How to properly index fields on a Rails join table migration?

Question

Rails 4 introduced a feature for generating join table migrations:

bin/rails generate migration CreateTeamsUsersJoinTable team user

This re

Answer 1

To complete xlembouras answer, you make your choice based on how you query your table.

For example, if you have a view that shows the users of a given team, the query will look like this :

SELECT user.name FROM team_users tu 
INNER JOIN users u 
ON tu.user_id = u.id
WHERE tu.team_id = X

This benefits from t.index [:team_id, :user_id] but not t.index [:user_id, :team_id]

The database will only use one index seek on the first index to retrieve the user ids

If you have a view that shows the teams of a given user, the query would be something like the following :

SELECT team.name FROM team_users tu 
INNER JOIN team t 
ON tu.team_id = t.id
WHERE u.user_id = X

This benefits from t.index [:user_id, :team_id] but not t.index [:team_id, :user_id]

The database will only use one index seek on the first index to retrieve the team ids

That's because of the way composed index are stored :