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

I would have to say that the intention behind this is to let you choose the strategy that fits you better. That is usually a decision with two criteria

• how much overhead does indexing adds
• how you'll query your db

So they can not make that decision for you and add the indexing overhead as a convention.

In order to make that decision you need to know how you intend to query your database. In MySQL composite indexes can be of use for a set of queries. multiple-column index

An index on column_a, column_b, column_c will be used for queries on

• all three fields together
• column_a alone
• column_a and column_b together

So there is a difference between

t.index [:team_id, :user_id]

and

t.index [:user_id, :team_id]

the actual full set of indexes that you 'll need is

t.index [:team_id, :user_id]
t.index :user_id

or

t.index [:user_id, :team_id]
t.index :team_id

In order to handle with an index all three cases.

example

if you find your app to use more frequently queries like

user.teams which translate to

select * from teams where user_id = X

an index on user_id (or user_id, team_id) can be handy

so t.index [:user_id, :team_id] should be your choice