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Quick Tech Tricks Technology Tips

Pi-Hole & DNS over HTTPS

There are some cool things about running your own DNS server. I use it to reference some web services that I run internally more easily, like my Unifi Controller, PiAware, and my home-grown Linux Server. It’s a lot easier than trying to keep track of the IP addresses of everything in your network.

For that reason, I’ve been running my own DNS server for awhile now. For awhile, I was just running my own BIND instance. Unfortunately, probably because I’m just not familiar with it, it wasn’t very stable. Luckily, something even better came along: Pi-Hole.

What’s Pi-Hole?

Pi-Hole is a stable, free, and lightweight DNS server with built-in adblocking abilities. The coolest part is that it blocks ads across your network (assuming you tell all your devices to use it as their DNS server), even devices that don’t support things like adblocking extensions. In my case, it’s the perfect solution.

My Old Approach

For awhile, I’ve been running Pi-Hole in Docker on my Linux server. To keep my DNS encrypted (because really, who wants to expose all their DNS requests to their ISP?), I’ve been routing queries over a VPN. The problem with this is that if, for some reason, my VPN connection fails, my DNS stops, which means all my devices think they’re not connected to the Internet. Obviously, that’s not ideal.

One other bit is that (probably through my own stupidity) my Docker container running Pi-Hole doesn’t automatically start up when I reboot the server. This has been a bother sometimes, even if it’s not something really serious. So I’ve decided to switch things up & figure out a way to make it more robust.

Pi-Hole on a Raspberry Pi

I’ve decided to start running Pi-Hole on a machine dedicated to the task. Since it’s designed for a Raspberry Pi, and I had an extra one (or two…) lying around, this seemed like a good way to go.

So I downloaded Raspbian, flashed it onto a Micro SD card, and touched a new file called ssh to enable SSH access without having to plug in a mouse, keyboard, and monitor. Then I went to my router’s control panel to find the IP address of the Raspberry Pi so I could ssh in. Raspbian boots up with username pi and password raspberry – log in & change your password before continuing (ssh pi@<IP ADDRESS> and then passwd pi to change it).

Finally, after changing the password, I ran through the Pi-Hole installation guide to get Pi-Hole itself set up. It’s about as simple and straightforward as it gets, honestly.

When you finish the installation, make sure you take note of the web admin password that it outputs – you’ll need it later. If you don’t, you’ll be able to reset it from the command line when you need it).

DNS over HTTPS

To encrypt our DNS queries outside the network, we’re gonna make Pi-Hole use cloudflared‘s DNS proxy feature. The nice thing about this is that this service will only handle DNS traffic, and since the Raspberry Pi is only running cloudflared and Pi-Hole, there’s a lot less risk of things interfering with it than on my server. Plus, running both Pi-Hole and cloudflared as services mean they start automatically and can automatically restart if they fail.

Installing cloudflared is pretty simple, since it’s a Golang binary. For a Raspberry Pi, download the ARM package from the Cloudflare Developers site. Assuming you’re using the command line, use these steps to download, extract, and move it into place:

wget https://bin.equinox.io/c/VdrWdbjqyF/cloudflared-stable-linux-arm.tgz
tar xzvf cloudflared-stable-linux-arm.tgz
sudo chmod +x ./cloudflared
sudo mv ./cloudflared /usr/local/bin

Then, to verify it’s working, run cloudflared -v – it should output the version (as of now, it’s 2019.12.0).

Run cloudflared as a Service

Next up, we need to set up cloudflared‘s proxy-dns command as a service. First up, create a new user to run the service:

sudo useradd -Mr -s /bin/nologin cloudflared

Then, change the permissions on the executable so that it’s owned by our new user:

sudo chown cloudflared:cloudflared /usr/local/bin/cloudflared

To actually make the service, we put the following into /etc/systemd/system/cloudflared.service:

[Unit]
Description=cloudflared DNS Proxy
After=syslog.target network-online.target

[Service]
Type=simple
User=cloudflared
ExecStart=/usr/local/bin/cloudflared proxy-dns --port=5053 --upstream https://1.1.1.1/dns-query --upstream https://1.0.0.1/dns-query
Restart=on-failure
RestartSec=5
KillMode=process

[Install]
WantedBy=multi-user.target

This will tell systemd how to run our service – we’re pointing it to Cloudflare’s https endpoints for DNS and telling it to listen on port 5053. It’ll restart automatically if it fails, and it won’t start up until after the network is online. Now we can start it up:

sudo systemctl enable cloudflared
sudo systemctl start cloudflared

If you run sudo systemctl status cloudflared, it should now show you active (running) in the output. To test that it’s working properly, run nslookup -port=5053 google.com 127.0.0.1 (from your Raspberry Pi). You should see something like this as output:

Server:         127.0.0.1
Address:        127.0.0.1#5053

Non-authoritative answer:
Name:   google.com
Address: 172.217.14.206
Name:   google.com
Address: 2607:f8b0:400a:803::200e

With this command, we’re telling nslookup to point to localhost on port 5053, which is where we’re running our cloudflared proxy. If you get an error, you’ll have to troubleshoot it.

Point Pi-Hole to Cloudflared

The last thing we need to do is tell Pi-Hole to use our cloudflared proxy as its DNS server so that all its DNS requests are encrypted by HTTPS. Access your Pi-Hole’s web interface by entering its IP address in your browser. Click ‘login’ on the left and enter the password that Pi-Hole output when you first installed it. If you lost the password (or didn’t write it down), you can reset it by entering pihole -a -p '<PASSWORD>' in the SSH console of your Raspberry Pi.

Then click ‘Settings’ in the left panel, then ‘DNS’ in the tabs at the top after that. In the resulting settings panel, uncheck all the boxes on the left-hand side, and enter 127.0.0.1#53 in the ‘Custom 1 (IPv4)’ box on the right. Make sure you check its box, too.

Then scroll down & click ‘Save’ at the bottom.

Finally, run a DNS query against your Pi-Hole (from another machine), putting your Pi’s IP address in:

> nslookup google.com <IP ADDRESS>
Server:         <IP ADDRESS>
Address:        <IP ADDRESS>#53

Non-authoritative answer:
Name:   google.com
Address: 172.217.14.238

If you get output like that ^, your Pi-Hole is correctly configured! Next up, make sure to make all your machines use your Pi-Hole’s IP address as their DNS server. The easiest way to do it network-wide is to set your DHCP server (normally your router) to use that IP as its only DNS server. Here’s what it looks like from my DHCP server config:

Once you’ve done that, your machines should start using the Pi-Hole as their DNS server as soon as they renew their DHCP lease. You can speed this along by forcing them to reconnect.

Once you’ve done that & they start using the Pi-Hole, your Pi-Hole Dashboard should start showing some traffic (and an increase in the number of clients), like this:

I hope this was helpful to you! I’ve you’ve got any questions, drop them in the comments below, and I’ll do my best to answer them.

Categories
Code Complete Technology

Code Complete Notes, Chapter 3, Section 5: Architecture

Architecture, according to the book (which draws from other sources as well), is:

Software architecture is the high-level part of software design, the frame that holds the more detailed parts of the design.

S. McConnell, Code Complete, second edition. Redmond (Washington): Microsoft Press, 2004.

Now that’s out of the way, let’s talk about how we can make sure that the architecture that’s laid out for our projects is good, since that’s what this chapter is all about. If your architecture is off at this stage, you’ll have to make the changes when you’re in the construction stage… That makes them a lot more difficult to solve, because you’ll likely have to move other things that you’ve already built around as you make the changes.

If you’re working on a system that was architected by someone else, you should be able to find the important components without a bloodhound, a deer-stalker cap, and a magnifying glass.

S. McConnell, Code Complete, second edition. Redmond (Washington): Microsoft Press, 2004.

The important parts of your architecture should be clearly visible. If it’s not clear why a class or component exists, or why it was chosen over other alternatives, then it’s possible it needs to be better defined.

Architectural Components

Photo by Iker Urteaga on Unsplash

I was originally going to outline the different components and give a few details on each, but then I realized I’d mostly just be paraphrasing the book. Since there’s no need to do that, here’s a list of the architectural components, along with some of my associated thoughts.

  • Program Organization
  • Major Classes
  • Data Design
  • Business Rules
  • User Interface Design
  • Resource Management
  • Security
  • Performance
  • Scalability
  • Interoperability
  • Internationalization & Localization
  • Input/Output
  • Error Processing
  • Fault Tolerance
  • Architectural Feasibility
  • Overengineering
  • Buy vs. Build Decisions
  • Reuse Decisions
  • Change Strategy
  • General Architectural Quality

Now, it seems to me that a lot of these, such as the User Interface Design, depend a lot on the type of development that you’re going for. If you’re developing in a waterfall-style approach, then it’s great to have it fully defined upfront. However, I think that most modern software development occurs much more iteratively – so perhaps these should all be thought of in shorter iterations.

If you’re developing using an ‘agile’ approach, it seems like it’d make sense to have these things laid out at the beginning of a sprint. But it all depends on the approach you’re using. I know I’ve never used a strict agile methodology, so you have to apply this where you see fit.

This is covered a bit in the ‘Change Strategy’ point, but I still don’t feel like it fully addresses more iterative development approaches. It’s possible I’m just missing something, though.

Conclusion

Anyway, the point is that for any development, you definitely should have at least a basic architecture in place. Even if you’re developing iteratively, think through the things in the list above so that you can define an overview of the system before you put your fingers to the keyboard & start coding.

It shouldn’t look as if the problem and the architecture have been forced together with duct tape.

S. McConnell, Code Complete, second edition. Redmond (Washington): Microsoft Press, 2004.
Categories
Code Complete Technology

Code Complete Notes, Chapter 3, Sections 2 and 3

Defining the Problem

Section 2 of Chapter 3 is very simple: you should know what problem you’re trying to solve before you try to solve it. It normally shouldn’t be stated in technical terms. Instead, it should be the simplest issue that you’re trying to solve.

For example, “We need developers to be able to parallelize their tests” isn’t a good problem definition, it’s already settled on a solution. “Developers’ tests take way too long to run” would be much better, because it states the problem, not a solution.

Defining Requirements

Section 3 covers devising the requirements for your software project. These should be agreed upon before work begins, and the customer should be the one in charge of validating the requirements.

Crafting well-defined requirements before starting the actual coding is important. It costs a lot more to adjust a design to meet new or updated requirements after code has been written than it does to just write the code to fit the requirements in the first place.

Specifying requirements adequately is a key to project success, perhaps even more important than effective construction techniques.

S. McConnell, Code Complete, second edition. Redmond (Washington): Microsoft Press, 2004.

But of course, requirements are rarely perfectly defined. It’s hard for customers (and developers, too!) to accurately describe requirements initially. As customers come to understand the system better, they’ll be better able to develop accurate requirements. So no matter how much we want things to be perfect the first time, it’s unlikely to ever happen in practice.

… the average project experiences about a 25 percent change in requirements during development.

S. McConnell, Code Complete, second edition. Redmond (Washington): Microsoft Press, 2004.

Dealing with changing requirements can be difficult, but it can be made easier by making it clear to your customer or client that changes cost, often monetary (though that depends on the type of project) and almost always in terms of time. If a customer is too happy-go-lucky with changes, establishing a change-control board to review the proposed changes may also be helpful (though I’d imagine that’s mostly where you’re doing client work).

One other important consideration is keeping the business reason for the project in mind. Oftentimes, you’ll find that features that sound neat or even necessary won’t be so important when you consider the main reason for the project.

Finally, this section concludes with a series of questions to ask yourself about your project’s requirements to ensure they’re solid. Reference the book directly for the list.

Categories
Code Complete Technology

Code Complete Notes, Chapter 3, Section 2

Different types of software require different types of planning. If you’re working on your own blog, for instance, the stakes are a lot lower than if you’re working on, say, an automated flight control system.

If you’re working on one of those high-stakes projects, your planning should be much more thorough & much less iterative. Your development is much more likely to follow a waterfall approach. But on your blog, or even something like an e-commerce site, you can (and arguably should) be a lot more iterative.

In either case, specifying more prerequisites upfront will help save time and effort in the long run, but it’s more important in more sequential projects. But, of course, your project’s balance between sequential and iterative will depend largely on what type of project it is and the risks involved.

One common rule of thumb is to plan to specify about 80 percent of the requirements up front…

S. McConnell, Code Complete, second edition. Redmond (Washington): Microsoft Press, 2004.

Essentially, you’ll need to pick the right balance for your project. If it’s something more stable, where the requirements are well understood and unlikely to change, a more sequential approach is appropriate. But if the requirements may change, or the structure isn’t as well understood, then a more iterative approach will be more beneficial.

Software being what it is, iterative approaches are useful much more often than sequential approaches are.

S. McConnell, Code Complete, second edition. Redmond (Washington): Microsoft Press, 2004.

For more details (as well as an analysis of the potential costs associated with each approach), check out this section of the book.

Categories
Code Complete Technology

Code Complete, Chapter 3, Section 1 Commentary

This is the second post in my series on Code Complete, covering my notes and commentary from Chapter 3, Section 2. The title of this section is Measure Twice, Cut Once: Upstream Prerequisites.

Essentially, this section is talking about the importance of developing prerequisites before beginning work on a project. It reminds me of a phrase that was apparently one of Frank Lloyd Wright’s favorites:

The architect’s two most important tools are: the eraser in the drafting room and the wrecking bar on the site.

Frank Lloyd Wright

In Software, this is equally applicable. The easier that we can catch defects, from design to development to production, the easier they are to fix. Obviously, in Wright’s quote above, using the eraser would be significantly cheaper than using a wrecking bar.

Much of the success or failure of the project has already been determined before construction begins.

S. McConnell, Code Complete, second edition. Redmond (Washington): Microsoft Press, 2004.

The diagram below is one that I created for a presentation given at Etsy – it’s a very generalized diagram, not representative of any actual set of data. However, it gives an idea of the relative cost to fix defects that are introduced during the software development process.

This section in Code Complete features a similar diagram – but backed by more solid data. Essentially, the earlier that defects are detected and corrected, the cheaper they are to fix.

…debugging and associated rework takes about 50 percent of the time spent in a typical software development cycle…

S. McConnell, Code Complete, second edition. Redmond (Washington): Microsoft Press, 2004.

So really, the whole point of this section is that planning reduces the overall cost of software development. Of course, this doesn’t mean that we should do everything in a waterfall approach and attempt to plan everything before we even start coding. We have to strike a balance between the two.

I believe, when using Agile methodologies, we should ensure that we plan carefully for the features that we’re focusing on building. Whether you’re using sprints or not, you can plan ahead for the work that you’re doing to ensure that you’re minimizing the ‘rework’ required.

It’s by striking a balance between planning too much and planning too little that we can be most effective in our software projects.

If you start the process with designs for a Pontiac Aztek, you can test it all you want to, and it will never turn into a Rolls-Royce.

S. McConnell, Code Complete, second edition. Redmond (Washington): Microsoft Press, 2004.
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Code Complete Technology

Code Complete, Chapters 1 and 2

Hello, everyone! I’ve recently started reading Code Complete (the second edition), by Steve McConnell. I haven’t made it very far into it yet, but I figured that I’d share the things that I learn or find interesting here.

I realized that when I read, it helps me to take notes and focus on highlighting so that I can increase my comprehension. So here I am, writing down some notes that I gather as I read Code Complete.

Software Construction

Something that was new to me was the idea of Software Construction. Software Construction encompasses coding, debugging, detailed design (not graphic design, mind you), testing, and integration. In essence, the things that we normally think of when we think of software development.

McConnell focuses Code Complete on Software Construction, rather than architecture, project management, or user interface design . He says that very few books released before the first edition of Code Complete had covered it directly.

It seems to me that a lot of books had touched on the topic of Construction, but it did seem like few had chosen to address it directly. Honestly, it feels like a majority of the books written regarding Software Development cover specific languages or frameworks. Of course, that could just be the fact that I never had a formal Computer Science education and therefore wasn’t exposed to that during my education. So that’s the value in this book – it’s trying to cover all the different aspects of Software Construction in one place.

Anyway, I don’t know why I hadn’t heard of Software Construction before, but I simply hadn’t. Really, though, the idea makes a lot of sense. McConnell also touches on the value of metaphors in understanding Software Construction, and it seems to me that just calling it Construction to begin with is itself a metaphor.

Metaphors

McConnell covers several different metaphors that software construction has been compared to over the years, including writing, farming, oysters (accretion), and construction. Of these, I found construction to be the most applicable.

Software must be architected first, just like buildings. Without a plan, your building/software isn’t going to end up very nice. Remodels are like refactors or adding additional functionality to software. Bigger buildings (or bigger software projects) require more planning than smaller projects.

One example that McConnell gives is that of building a dog house. You don’t really need to plan much ahead to build a dog house. Likewise, a tiny software project probably doesn’t require much in the way of architecture. But if you’re building a skyscraper, that takes an awful lot more work and planning. You wouldn’t just want to go to the hardware store & pick up some random materials for your skyscraper, but that’s feasible in the case of a dog house.

The one area where I think this breaks down some (but not entirely) is in refactoring and adding additional features. It’s still a lot easier to work on software than it is to add on to a house. A lot of software, especially web-based software, can be continually improved. That’s more difficult to do with a building. Not impossible, just more difficult.

Anyway, that covers my thoughts on Chapters 1 and 2 of Code Complete. Keep your eyes peeled for more posts covering my thoughts on the book.

Categories
Quick Tech Tricks

Using `xarg` to pass to `find`

I just found myself needing to run wc -l on all the files in a list of directories – in my case, I had a big old list of directories with a matching name. But I wanted to calculate the total number of files in those directories.

Unfortunately, find is very particular about where its arguments go, so running xargs and passing it to find was resulting in the following:

find: paths must precede expression: tmp/dir/name
Usage: find [-H] [-L] [-P] [-Olevel] [-D help|tree|search|stat|rates|opt|exec] [path...] [expression]

Luckily, the solution is pretty simple. Use the -I flag to xargs to make it replace {} with your argument.

So your full command will look something like this:

cat list_of_directories.txt | xargs -I{} find {} | whatever

The command that I ended up with is this:

find . -name "dir_name" | xargs -I{} find {} -type f | wc -l

I hope that’s helpful for you.

(Reference: http://xion.io/post/code/shell-xargs-into-find.html)

Categories
Automation Configuration Technology

Creating and Applying Diffs with Rsync

At work recently, we had a need to generate diffs between two different directory trees. This is so that we can handle deploys, but it’s after we’ve already generated assets, so we can’t just use git for the diff creation, since git diff doesn’t handle files that aren’t tracked by git itself. We looked into using GNU’s diffutils, but it doesn’t handle binary files.

We tried investigating other methods for deploying our code, but thought it would still be simplest if there was some way to generate just a ‘patch’ of what had changed.

Luckily, one of the Staff Engineers at Etsy happened to know that rsync had just such an option hiding in its very long man page. Because rsync handles transferring files from one place to another, whether it’s local or remote, it has to figure out the diffs between files anyway. It’s really nice that they’ve exposed it so that you can use the diffs themselves. The option that does this is called ‘Batch Mode’, because you can use it to ‘apply’ a diff on many machines after you’ve distributed the diff file.

Creating the Diff

To create the diff itself, you’ll need to first have two directories containing your folder structure – one with the ‘previous’ version and one with the ‘current’ version. In our case, after we run each deploy, we create a copy of the current directory so that we can use that as our previous version to build our next diff.

Your rsync command will look a lot like this:

rsync --write-batch=diff /deploy/current /deploy/previous

Running that command will give you two files, diff and diff.sh. You can just use the .sh file to apply your diff, but you don’t have to. As long as you remember to use the same flags when applying your diff, you’ll be fine. You can also use any filename that you want after the =.

Also, it’s important to note that running this command will update /deploy/previous to the contents of /deploy/current. If you want to keep /deploy/previous as-is so that you can update it later, use --only-write-batch instead of just --write-batch.

Applying the Diff

Next up, you’ll want to distribute your diff to whatever hosts are going to receive it. In our case, we’re uploading it to Google Cloud Storage, where all the hosts can just grab it as necessary.

On each host that’s applying the diff, you’ll want to just run something like the following:

rsync --read-diff=/path/to/diff /deploy/directory

Remember, you need to use the same flags when applying your diff as you did when you created your diff.

In our testing, this worked well for applying a diff to many hosts – updating around 400 hosts in just about 1 minute (including downloading the ~30MB diff file to each host).

Caveats

This will fail if the diff doesn’t apply cleanly. So, essentially, if one of your hosts is a deploy behind, you should make absolutely sure that you know that, and don’t try to update it to the latest version. If you try to anyway, you’ll probably end up with errors in the best case, or a corrupt copy of your code in the worst case. We’re still working on making our scripts handle the potential error cases so that we don’t end up in a corrupt state.

I hope this is helpful to you! If you’ve got any thoughts, questions, or corrections, drop them in the comments below. I’d love to hear them!

Categories
Ruby Technology

Saving Calculated Fields in Ruby on Rails 5

In Ruby on Rails, it’s easy to build custom functions to calculate something and then display the result in your views. While this simplicity is nice, it doesn’t come without its drawbacks.

Recently, when working on a simple app, I came across a situation where loading a page was taking 0.5 seconds. This may not sound like a lot (and wouldn’t be for most sites), but in an app as simple as mine, it’s a sign that something is taking way longer than it should. Luckily, it wasn’t too difficult to determine what it was.

The Problem

Let’s start with an example: say you’re building an application that will contain purchases from a grocery store. You probably want to link the items sold in a purchase with the record from that purchase, right? Well, somewhere you’re going to have to calculate the total. Of course, I’m assuming that you don’t want the customer to calculate the total.

You could calculate the total every time that you need to load the record of the purchase, but first let’s walk through what would be happening when you calculated the total. If there are, say, 30 items in that purchase, you’ll need to load every single one of those items so that you can grab the price (we’re assuming prices don’t change for this example) and add them all together.

As you might imagine, this isn’t a very efficient way to go about things. We’d rather offload some of that computation (that would be happening an awful lot) to the disk, instead. After all, it’s generally easier to store a few bytes than spend valuable CPU time recalculating it every time you need it.

In my case, that’s exactly the sort of thing that was happening. I was working to calculate a field that wouldn’t change often but that involved loading lots of links to other records. On top of that, it was going to be loaded pretty often. It’s much more efficient for me to just store that value than to calculate it for every request.

The Solution

You’ll need to add a new field to your database, which means you’ll need to add a database migration, something like this:

rails generate migration AddTotalToReceipts total:float

After you run your migration (rails db migrate), you’ll have your new field. Now, if you generated all your scaffolding, that’d be showing up in your user interface. That’s not what you want to do, though, since we’re trying to make this easier for your users and calculate it on their behalf.

Thus, we’re going to add something like the following to our model:

before_save :calculate_receipt_total

def calculate_receipt_total
  sum_value = x + y # Whatever you need to do here to calculate
  self.total = sum_value
end

Now that method will run automatically before the record is saved, and place our calculated value into the total value, which means it’ll end up there in the database, as well.

Like I said, just how much benefit (if any) you’ll get out of this depends on your exact circumstances, but in my case it reduced a 500 ms page load to around 100 ms, which is clearly a substantial improvement.

If you’ve got any questions, drop them in the comments, and I’ll do my best to answer them!

Categories
Ruby Technology

Many-to-Many in the UI in Ruby on Rails 5

I know I’m really late to the party, but I’m just finally creating something from scratch in Ruby on Rails. I’ve dealt a tiny bit with Rails before, but it was mostly just in helping my brother with CSS stuff, which obviously isn’t working on the Rails backend.

Anyway, now that I’ve started working with Rails (to build a simple app for my wife), I’ve found myself needing to learn how to do things in Rails 5. The problem is, a lot of things have changed in Rails, but most Google search results for ‘Rails …’ or ‘Ruby on Rails …’ end up with articles that are at least several years old. It’s hard to figure out what actually applies to Rails 5, vs. any other version of Rails. Thus, I’ve decided to write up some of my findings so that they’re hopefully helpful for someone else.

I’ll assume that you already know how to create your initial models, views, and controllers. If you don’t, check out this guide to get started.

Creating Many:Many Associations

Creating links between your records is pretty straightforward. I’m going to be using hypothetical ‘products’ and ‘purchases’ tables, which aren’t necessarily a perfect use-case for this, but they’re good enough. We can use Rails’s simple generator to make a link between our tables:

rails generate migration CreateJoinTableProductsPurchases

All that’s going to do is generate a migration file (if you want those indexes, uncomment the two t.index lines):

class CreateJoinTableProductsPurchases < ActiveRecord::Migration[5.2]
  def change
    create_join_table :products, :purchases do |t|
      # t.index [:product_id, :purchase_id]
      # t.index [:purchase_id, :product_id]
    end
  end
end

After updating our database with the migration (rails db:migrate), we just need to add a has_and_belongs_to_many to each of our models:

class Product < ApplicationRecord
    has_and_belongs_to_many :purchases
end
class Purchase < ApplicationRecord
    has_and_belongs_to_many :product
end

I know that using has_and_belongs_to_many may be going out of favor, but I haven’t had time to look at the alternative just yet, so I’m sticking with HABTM for now.

Creating Many:Many Links in the UI

Now you’ve got a link between your two models, but no convenient way to create any links! That’s what we’ll be focusing on next. Luckily, it’s pretty easy, you just need to know what to do.

Showing Many:Many Forms

First off, in the form for the ‘products’ side of our relationship, we’ll simply add the following:

<%= form.label :products %>
<%= collection_select(:purchase, :product_ids, Product.all, :id, :name, {}, { :multiple => true } )%>

If you’re using Rails’s scaffold generator, you’ll add it to app/views/<model_name>/_form.html.erb. This creates a list of all the ‘products’ (in the example code) in the form for a purchase, and allows you to select multiple products.

If you’d like, you can add similar code to the other side of your relationship. In my case, I’m just adding it on one side. The form will end up looking something like this:

You can use ‘Ctrl/Cmd+click’ to select more than one item in the list, but that’s really all there is to it. It’s a very basic form, but it’ll do the trick.

Of course, you can do something more advanced, but we’re just covering the basics here. Ideally, I’d like a multi-select autocompleting textbox. Hopefully some day in the near future I’ll be able to put a guide up here on how to do that.

Saving Many:Many Relationships

Now, to make our selections actually save, we’ve got to add something to the controller. Since I’m only allowing the creation of links from the ‘Purchase’ UI, I put this code in my app/controllers/purchases_controller.rb file, in both the create and the update methods:

params[:purchase][:product_ids].each do |product_id|
  unless product_id.empty?
  product = Product.find(product_id)
    @boarding.products << product
  end
end

And that’s all there is to it! When you either create or update a ‘purchase’, your selection for its ‘products’ will be saved.

Viewing Many:Many Links in the UI

So now that we’re able to easily create links, it would be nice if we could view those links. So let’s make it possible to see the list of ‘products’ in a ‘purchase’. Using the Rails scaffolding, we’re going to want to expose these in both the ‘list’ view and the ‘single’ view for each side of our link. You can pick if the same applies to you.

In our app/views/purchases/index.html.erb, we’re going to add the following to a new <td> that will display the ‘products’ in each ‘purchase’. Don’t forget to add an associated <th> in the header, as well.

<%= purchase.products.map(&:name).join(', ') %>

Then, upon loading /purchases, you’ll see something a lot like this:

Next up, we need to add some very similar code to our app/views/purchases/show.html.erb file (but note the @ in the name of the purchase variable:

<%= @purchase.products.map(&:name).join(', ') %>

And then you’ll have something like this when you view your ‘purchase’ (at a URL like /purchases/2):

Conclusion

That’s it! As I said, this isn’t a perfect use-case for a many:many relationship, but it’s at least an overview of how to create the UI elements that will allow you to save and view such relationships.

If you want to take a look at the code in its entirety, it’s available on GitHub. And, if you’ve got any tips or comments, drop them in the ‘comments’ section below! That’s what it’s there for!