Subqueries in activerecord

Solution 1:

Rails now does this by default :)

Message.where(user_id: Profile.select("user_id").where(gender: 'm'))

will produce the following SQL

SELECT "messages".* FROM "messages" WHERE "messages"."user_id" IN (SELECT user_id FROM "profiles" WHERE "profiles"."gender" = 'm')

(the version number that "now" refers to is most likely 3.2)

Solution 2:

In ARel, the where() methods can take arrays as arguments that will generate a "WHERE id IN..." query. So what you have written is along the right lines.

For example, the following ARel code:

User.where(:id => Order.where(:user_id => 5)).to_sql

... which is equivalent to:

User.where(:id => [5, 1, 2, 3]).to_sql

... would output the following SQL on a PostgreSQL database:

SELECT "users".* FROM "users" WHERE "users"."id" IN (5, 1, 2, 3)" 

Update: in response to comments

Okay, so I misunderstood the question. I believe that you want the sub-query to explicitly list the column names that are to be selected in order to not hit the database with two queries (which is what ActiveRecord does in the simplest case).

You can use project for the select in your sub-select:

accounts = Account.arel_table
User.where(:id => accounts.project(:user_id).where(accounts[:user_id].not_eq(6)))

... which would produce the following SQL:

SELECT "users".* FROM "users" WHERE "users"."id" IN (SELECT user_id FROM "accounts" WHERE "accounts"."user_id" != 6)

I sincerely hope that I have given you what you wanted this time!

Solution 3:

I was looking for the answer to this question myself, and I came up with an alternative approach. I just thought I'd share it - hope it helps someone! :)

# 1. Build you subquery with AREL.
subquery = Account.where(...).select(:id)
# 2. Use the AREL object in your query by converting it into a SQL string
query = User.where("users.account_id IN (#{subquery.to_sql})")

Bingo! Bango!

Works with Rails 3.1