Programming Ecto

Working with Associations in Queries

Now that we’ve got some associations defined, let’s put them to work. Go to the music_db project and open up a mix session with iex -S mix.

First grab the record for the album Kind Of Blue:

priv/examples/schema_10.exs

album = Repo.get_by(Album, title: "Kind Of Blue")

Our gut tells us that if we want to see the tracks for this album, we would just do this:

album.tracks

But our gut is wrong. Instead, we get this:

#Ecto.Association.NotLoaded<association :tracks is not loaded>

This is not an error. It’s a placeholder value indicating that the tracks records associated with this album have not yet been retrieved from the database.

You might well wonder why Ecto doesn’t just load the ...