This was TIL for me, so I thought it would be worth a quick post. 🙂
TL; DR;
Repo.all from u in User,
preload: [{:investiment_funds, :institution}, :transfers]
Ecto is designed to not bring associations alongside the main data in the query to avoid N+1 problems.
If you want, you need to explicitly say what is that you want to bring together in the query.
To do that you need to use the preload macro to load the associations into the given query.
Imagine you are at investment scenario where you want to fetch all users and list their investment funds, you would need something like that to preload the investment funds.
Repo.all from u in User,
preload: [:investiment_funds]
Now, imagine you also need to list the transfers that the user did.
Repo.all from u in User,
preload: [:investiment_funds, :transfers]
No problem so far, this is all covered by the Ecto documentation.
But now imagine you need to show in the investment funds list, the fund institution that is stored in a different table.
User -> Investment Fund -> Institution
You don’t need to use joins or anything like that in order to make it work, you will only need to pass the nested associations inside the curly brackets and everything should work now.
Repo.all from u in User,
preload: [{:investiment_funds, :institution}, :transfers]
Hope it helps!
References: