What's the difference between “includes” and “joins” in ActiveRecord query?

Question

What\'s the difference between \"includes\" and \"joins\" in ActiveRecord query? Can anyone explain to me with the following two associated

Answer 1

TL; DR

Joins:

a.joins(:b).to_sql
=> "SELECT \"a\".* FROM \"a\" INNER JOIN \"b\" ON \"b\".\"id\" = \"a\".\"b_id\""

Includes:

a.includes(:b).to_sql
=> "SELECT \"a\".* FROM \"a\"

Both:

a.includes(:b).joins(:b).to_sql
=> "SELECT \"a\".\"id\" AS t0_r0, \"a\".\"a_field_1\" AS t0_r1, \"a\".\"a_field_2\" AS t0_r2, \"a\".\"a_field_3\" AS t0_r3, \"b\".\"a_field_1\" AS t1_r1, \"b\".\"a_field_2\" AS t1_r2, \"b\".\"a_field_3\" AS t1_r3 FROM \"a\" INNER JOIN \"b\" ON \"b\".\"id\" = \"a\".\"plan_id\""