LEFT OUTER JOIN in Rails 4

Question

I have 3 models:

class Student < ActiveRecord::Base
  has_many :student_enrollments, dependent: :destroy
  has_many :courses, through: :student_enrollments
end

class Course < ActiveRecord::Base   
    has_many :student_enrollments, dependent: :destroy
    has_many :students, through: :student_enrollments
end

class StudentEnrollment < ActiveRecord::Base
    belongs_to :student
    belongs_to :course
end

I wish to query for a list of courses in the Courses table, that do not exist in the StudentEnrollments table that are associated with a certain student.

I found that perhaps Left Join is the way to go, but it seems that joins() in rails only accept a table as argument. The SQL query that I think would do what I want is:

SELECT *
FROM Courses c LEFT JOIN StudentEnrollment se ON c.id = se.course_id
WHERE se.id IS NULL AND se.student_id = <SOME_STUDENT_ID_VALUE> and c.active = true

How do I execute this query the Rails 4 way?

Any input is appreciated.

Answer 1

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")

Answer 2

If anyone came here looking for a generic way to do a left outer join in Rails 5, you can use the #left_outer_joins function.

Multi-join example:

Ruby:

Source.
 select('sources.id', 'count(metrics.id)').
 left_outer_joins(:metrics).
 joins(:port).
 where('ports.auto_delete = ?', true).
 group('sources.id').
 having('count(metrics.id) = 0').
 all

SQL:

SELECT sources.id, count(metrics.id)
  FROM "sources"
  INNER JOIN "ports" ON "ports"."id" = "sources"."port_id"
  LEFT OUTER JOIN "metrics" ON "metrics"."source_id" = "sources"."id"
  WHERE (ports.auto_delete = 't')
  GROUP BY sources.id
  HAVING (count(metrics.id) = 0)
  ORDER BY "sources"."id" ASC

Answer 3

There is actually a "Rails Way" to do this.

You could use Arel, which is what Rails uses to construct queries for ActiveRecrods

I would wrap it in method so that you can call it nicely and pass in whatever argument you would like, something like:

class Course < ActiveRecord::Base
  ....
  def left_join_student_enrollments(some_user)
    courses = Course.arel_table
    student_entrollments = StudentEnrollment.arel_table

    enrollments = courses.join(student_enrollments, Arel::Nodes::OuterJoin).
                  on(courses[:id].eq(student_enrollments[:course_id])).
                  join_sources

    joins(enrollments).where(
      student_enrollments: {student_id: some_user.id, id: nil},
      active: true
    )
  end
  ....
end

There is also the quick (and slightly dirty) way that many use

Course.eager_load(:students).where(
    student_enrollments: {student_id: some_user.id, id: nil}, 
    active: true
)

eager_load works great, it just has the "side effect" of loding models in memory that you might not need (like in your case)
Please see Rails ActiveRecord::QueryMethods .eager_load
It does exactly what you are asking in a neat way.

Answer 4

Combining includes and where results in ActiveRecord performing a LEFT OUTER JOIN behind the scenes (without the where this would generate the normal set of two queries).

So you could do something like:

Course.includes(:student_enrollments).where(student_enrollments: { course_id: nil })

Docs here: http://guides.rubyonrails.org/active_record_querying.html#specifying-conditions-on-eager-loaded-associations

Answer 5

Adding to the answer above, to use includes, if you want an OUTER JOIN without referencing the table in the where (like id being nil) or the reference is in a string you can use references. That would look like this:

Course.includes(:student_enrollments).references(:student_enrollments)

or

Course.includes(:student_enrollments).references(:student_enrollments).where('student_enrollments.id = ?', nil)

http://api.rubyonrails.org/classes/ActiveRecord/QueryMethods.html#method-i-references

Answer 6

You'd execute the query as:

Course.joins('LEFT JOIN student_enrollment on courses.id = student_enrollment.course_id')
      .where(active: true, student_enrollments: { student_id: SOME_VALUE, id: nil })

Answer 7

I know that this is an old question and an old thread but in Rails 5, you could simply do

Course.left_outer_joins(:student_enrollments)

Answer 8

You could use left_joins gem, which backports left_joins method from Rails 5 for Rails 4 and 3.

Course.left_joins(:student_enrollments)
      .where('student_enrollments.id' => nil)

Answer 9

I've been struggling with this kind of problem for quite some while, and decided to do something to solve it once and for all. I published a Gist that addresses this issue: https://gist.github.com/nerde/b867cd87d580e97549f2

I created a little AR hack that uses Arel Table to dynamically build the left joins for you, without having to write raw SQL in your code:

class ActiveRecord::Base
  # Does a left join through an association. Usage:
  #
  #     Book.left_join(:category)
  #     # SELECT "books".* FROM "books"
  #     # LEFT OUTER JOIN "categories"
  #     # ON "books"."category_id" = "categories"."id"
  #
  # It also works through association's associations, like `joins` does:
  #
  #     Book.left_join(category: :master_category)
  def self.left_join(*columns)
    _do_left_join columns.compact.flatten
  end

  private

  def self._do_left_join(column, this = self) # :nodoc:
    collection = self
    if column.is_a? Array
      column.each do |col|
        collection = collection._do_left_join(col, this)
      end
    elsif column.is_a? Hash
      column.each do |key, value|
        assoc = this.reflect_on_association(key)
        raise "#{this} has no association: #{key}." unless assoc
        collection = collection._left_join(assoc)
        collection = collection._do_left_join value, assoc.klass
      end
    else
      assoc = this.reflect_on_association(column)
      raise "#{this} has no association: #{column}." unless assoc
      collection = collection._left_join(assoc)
    end
    collection
  end

  def self._left_join(assoc) # :nodoc:
    source = assoc.active_record.arel_table
    pk = assoc.association_primary_key.to_sym
    joins source.join(assoc.klass.arel_table,
      Arel::Nodes::OuterJoin).on(source[assoc.foreign_key].eq(
        assoc.klass.arel_table[pk])).join_sources
  end
end

Hope it helps.

Answer 10

See below my original post to this question.

Since then, I have implemented my own .left_joins() for ActiveRecord v4.0.x (sorry, my app is frozen at this version so I've had no need to port it to other versions):

In file app/models/concerns/active_record_extensions.rb, put the following:

module ActiveRecordBaseExtensions
    extend ActiveSupport::Concern

    def left_joins(*args)
        self.class.left_joins(args)
    end

    module ClassMethods
        def left_joins(*args)
            all.left_joins(args)
        end
    end
end

module ActiveRecordRelationExtensions
    extend ActiveSupport::Concern

    # a #left_joins implementation for Rails 4.0 (WARNING: this uses Rails 4.0 internals
    # and so probably only works for Rails 4.0; it'll probably need to be modified if
    # upgrading to a new Rails version, and will be obsolete in Rails 5 since it has its
    # own #left_joins implementation)
    def left_joins(*args)
        eager_load(args).construct_relation_for_association_calculations
    end
end

ActiveRecord::Base.send(:include, ActiveRecordBaseExtensions)
ActiveRecord::Relation.send(:include, ActiveRecordRelationExtensions)

Now I can use .left_joins() everywhere I'd normally use .joins().

----------------- ORIGINAL POST BELOW -----------------

If you want OUTER JOINs without all the extra eagerly loaded ActiveRecord objects, use .pluck(:id) after .eager_load() to abort the eager load while preserving the OUTER JOIN. Using .pluck(:id) thwarts eager loading because the column name aliases (items.location AS t1_r9, for example) disappear from the generated query when used (these independently named fields are used to instantiate all the eagerly loaded ActiveRecord objects).

A disadvantage of this approach is that you then need to run a second query to pull in the desired ActiveRecord objects identified in the first query:

# first query
idents = Course
    .eager_load(:students)  # eager load for OUTER JOIN
    .where(
        student_enrollments: {student_id: some_user.id, id: nil}, 
        active: true
    )
    .distinct
    .pluck(:id)  # abort eager loading but preserve OUTER JOIN

# second query
Course.where(id: idents)

Answer 11

It'a join query in Active Model in Rails.

Please click here for More info about Active Model Query Format.

@course= Course.joins("LEFT OUTER JOIN StudentEnrollment 
     ON StudentEnrollment .id = Courses.user_id").
     where("StudentEnrollment .id IS NULL AND StudentEnrollment .student_id = 
    <SOME_STUDENT_ID_VALUE> and Courses.active = true").select

Answer 12

Use Squeel:

Person.joins{articles.inner}
Person.joins{articles.outer}

Answer 13

If anyone out there still needs true left_outer_joins support in Rails 4.2 then if you install the gem "brick" on Rails 4.2.0 or later it automatically adds the Rails 5.0 implementation of left_outer_joins. You would probably want to turn off the rest of its functionality, that is unless you want an automatic "admin panel" kind of thing available in your app!