Left Outer Join in Rails 4

LEFT OUTER JOIN in Rails 4

You can pass a string that is the join-sql too. eg joins("LEFT JOIN StudentEnrollment se ON c.id = se.course_id")

Though I'd use rails-standard table naming for clarity:

joins("LEFT JOIN student_enrollments ON courses.id = student_enrollments.course_id")

Left join with rails 4

You will have to write the left join yourself.

Section.joins("left join questions on questions.section_id = sections.id")
  .joins("left join answers on answers.question_id = questions.id")

Rails 4 Left outer join multiple tables with Group By and count

You could drop to SQL strings:

joins("LEFT OUTER JOIN comments ON comments.cuuser_id = cuuser.id LEFT OUTER JOIN likes ON likes.cuuser_id = cuuser.id LEFT OUTER JOIN messages on messages.cuuser_id = cuuser.id")

This isn't great. You're sacrificing some portability and the ability of ActiveRecord to guess the association.

If you used the Squeel gem you could use the following format:

joins{ [comments.outer, likes.outer, messages.outer] }

Squeel exposes Arel in a slightly more sane format, so you can do things like left outer joins while still guessing associations from the model definitions.

You could use Arel of course, but things get very clunky very quickly.

To get your counts, try:

select('cuusers.id, count(messages.id) as message_count, count(likes.id) as likes_count, count(comments.id) as comments_count').

They should be available as attributes on the returned objects, just like ordinary database fields.

How can I do a LEFT OUTER JOIN using Rails ActiveRecord?

I believe an includes will use a LEFT OUTER JOIN query if you do a condition on the table that the includes association uses:

Group.includes(:user_group_cmb).where(user_group_cmbs: { user_id: 1 })

How to left outer joins with conditions

Because you have a general where condition, you can use includes. This will generate a LEFT OUTER JOIN query:

Action.includes(:actions_users).where(actions_users: { user_id: true })

Or if you are using Rails 5+, Active Record provides a finder method left_outer_joins:

Action.left_outer_joins(:actions_users).where(actions_users: { user_id: true })

Multiple Left join with rails 4

You can do the following:

Section.joins(questions: :answer).where(answers: { user_id: USER_ID })

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Some stuff to know:

• In the joins/includes method, always use the exact same name as the relation
• In the where clauses, always use the pluralized name of the relation (actually, the table's name, which is by default the model name in plural but can also be manually set)

Examples:

# Consider these relations:
User has_many :posts
Post belongs_to :user

# Usage of joins/includes & where:
User.includes(:posts).where(posts: { name: 'BlogPost #1' })
                  #^            ^
Post.joins(:user).where(users: { name: 'Little Boby Table' })
              #^^           ^

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