Question

I have two models merchant and category with a HABM relationship so have a joining table called categories_merchants

How can I write the following query in ActiveRecords?

SELECT
    categories_merchants.merchant_id
WHERE
    categories_merchants.category_id IN (1,2,3,4)

NOTE: The joining table doesn’t have a model, it works automatically in Rails 3 without need for one.

Answer 1

If you want to do such a query you should transform your HABTM in a has_many :trough and create a model for the joining table and query based on that.

Alternatively, but possibly with worse performance, you could do:

Merchant
  .select(:id)
  .joins("categories_merchants ON categories_merchants.merchant_id = merchant.id")
  .where("categories_merchants.category_id IN (?)", [1,2,3,4])