Rails 4의 LEFT OUTER JOIN

Rails 4의 LEFT OUTER JOIN

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3 가지 모델이 있습니다.

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

특정 학생과 관련된 StudentEnrollments 테이블에없는 Courses 테이블의 강좌 목록을 쿼리하고 싶습니다.

아마도 Left Join이 갈 길이라는 것을 알았지 만 레일의 joins ()는 테이블을 인수로만 받아들이는 것 같습니다. 내가 원하는 것을 할 것이라고 생각하는 SQL 쿼리는 다음과 같습니다.

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

이 쿼리를 Rails 4 방식으로 어떻게 실행합니까?

모든 입력을 부탁드립니다.

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join-sql 인 문자열도 전달할 수 있습니다. 예 :joins("LEFT JOIN StudentEnrollment se ON c.id = se.course_id")

명확성을 위해 레일 표준 테이블 명명을 사용하지만 :

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

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Rails 5에서 왼쪽 외부 조인을 수행하는 일반적인 방법을 찾는 사람이 있다면이 #left_outer_joins함수를 사용할 수 있습니다 .

다중 조인 예 :

루비:

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

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실제로 이것을하기위한 "Rails Way"가 있습니다.

Arel을 사용할 수 있습니다 . 이는 Rails가 ActiveRecrods에 대한 쿼리를 생성하는 데 사용하는 것입니다.

나는 그것을 메서드로 감싸서 멋지게 호출하고 원하는 인수를 전달할 수 있습니다.

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

또한 많은 사람들이 사용하는 빠르고 (약간 더러운) 방법이 있습니다.

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

eager_load 그것은 당신이 (귀하의 경우처럼)가 필요하지 않을 수도 메모리에 loding 모델의 "부작용"을 가지고, 좋은 작품
레일 액티브을 참조하십시오 : QueryMethods는 .eager_load
그것은 당신이 깔끔한 방법으로 요구하고 정확하게 수행합니다.

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결합 includes하고 whereActiveRecord가 LEFT OUTER JOIN을 수행하는 결과를 얻습니다 (일반적인 두 쿼리 집합을 생성하는 위치없이).

따라서 다음과 같이 할 수 있습니다.

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

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

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위의 대답에 추가 includes하여을 사용 하려면 where (id가 nil 인 것과 같이) 테이블을 참조하지 않고 OUTER JOIN을 원하거나 참조가 문자열에 있습니다 references. 다음과 같이 표시됩니다.

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

또는

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

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

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쿼리를 다음과 같이 실행합니다.

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

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나는 이것이 오래된 질문이고 오래된 스레드라는 것을 알고 있지만 Rails 5에서는 간단하게 할 수 있습니다.

Course.left_outer_joins(:student_enrollments)

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Rails 4와 3을 위해 Rails 5의 메소드 를 백 포트 하는 left_joins gem을 사용할 수 있습니다 left_joins.

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

---

Rails의 Active Model의 조인 쿼리입니다.

활성 모델 쿼리 형식에 대한 자세한 내용을 보려면 여기를 클릭하십시오 .

@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

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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.

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Use Squeel:

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

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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)

참고URL : https://stackoverflow.com/questions/24358805/left-outer-join-in-rails-4