Your First Migration

Prerequisites

To get a feel for how to write migrations, we’ll add a new table to our MusicDB project. We’ve been using the tracks table to keep track of recordings of songs that have appeared on specific albums. However, many artists record songs that they did not write themselves, and certain pieces appear on more than one album. For example, the song “Freddie Freeloader” appears on two different albums in our current dataset. We’ll create a compositions table to track the metadata of the songs that appear in our dataset and later link them to specific tracks records.

Create a new migration

The easiest way to create a new migration is to use the mix task that Ecto provides:

mix ecto.gen.migration

This task has one required argument—the name of the migration we want to create.

Ecto will use this name in the migration module it will create for us. It’s best to make this name as descriptive as possible so that future developers can understand what the migrations do just by looking at the names. We’re going to create a new compositions table, so we’ll ...