There are some things I find confusing about Active Record ORM/DSL (ORM or DSL or both?). I’m gonna write them down here so next time I get confused about them I have a place to go to and look them up:

1. .joins is in fact an INNER JOIN

2. If you want a LEFT JOIN you need to use .left_joins

Which is also the same as .left_outer_joins Rails docs for .left_joins

3. If you want a RIGHT JOIN you flip the .left_joins

Are there other ways?

4. If you want to join multiple associations, you use the comma

  Book.joins(:author, :reviews)

Which produces:

  SELECT books.* FROM books
  INNER JOIN authors ON authors.id = books.author_id
  INNER JOIN reviews ON reviews.book_id = books.id

Reference

5. If you want a NESTED association, you do NOT use the comma

  Book.joins(reviews: :customer)

Which produces:

  SELECT books.* FROM books
  INNER JOIN reviews ON reviews.book_id = books.id
  INNER JOIN customers ON customers.id = reviews.customer_id

Reference
And following paragraphs.

6. Use .includes to help reduce N+1 queries, and eager load

  Customer.includes(:orders, :reviews)

This loads all the customers and the associated orders and reviews for each.

Reference
And following paragraphs.

General post references: