People starting out with ecto are often confused by all the methods of getting changes applied to a changeset and therefore the database. Especially the options around assocications are often problematic. To bring some clarity into the matter one first needs to understand a thing about the architecture ecto and changesets operate under.

There are three conceptional types of data ecto deals with. There’s user input, which is often provided in a subset of data types of what a schema is meant to carry. E.g. in a web context forms often submit data with text values only. JSON serialize data might know a few more data types, but is still quite limited. Then there’s the actual values one want’s to have have at runtime in schemas – your field :some_name, :date being a %Date{}. The third type is the data type the runtime data is serialized into for database storage.

Converting user input into...

On the Slack Channel for Elixir there’s often the situation of people coming from other languages/frameworks to elixir and especially ecto to ask about “How to model many-to-many relationships”.

Most of the times the usual answer comes as a surprise to people, as usage of Ecto.Schema.many_to_many is almost actively discouraged. I’ll try to bring some insights into why this is the case and what’s the better alternative.

many_to_many in ecto

For quite some time ecto didn’t even come with many_to_many relationships and people instead modeled them explicitly using plain belongs_to/has_many relationships on all tables involved. But due to popular demand explicit support was eventually added. (1)

There’s one caveat though. It was added with the intent of hiding away the implementation detail of needing a join-table to create the relationship between the two many-to-many schemas. Because...

I’ve thought that @rails deployment on a non-Docker or non-Heroku production server was tricky. But that is nothing compared to @elixirphoenix.

A million buggy how-tos and deployment tools. Not a single one works. Not a single README works. Always something “small” is missing.

— Stefan Wintermeyer (@wintermeyer) March 17, 2020

At the end of a two day hunt for a https://t.co/nY1mnbHhzA like tool I ended up with the advise of pros in an Elixir Slack channel to “just write a Bash script which glues everything together”.

— Stefan Wintermeyer (@wintermeyer) March 17, 2020

Thoughts like this are not really rare for people new to elixir. And they’re not totally wrong in their assessment either. Doing such a comparison however expects that a similar level of “just handle this for me” is actually possible for elixir tools to provide and if so that it’s a good idea to do so. This is an...