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# frozen_string_literal: true
module FromUnion
extend ActiveSupport::Concern
class_methods do
# Produces a query that uses a FROM to select data using a UNION.
#
# Using a FROM for a UNION has in the past lead to better query plans. As
# such, we generally recommend this pattern instead of using a WHERE IN.
#
# Example:
# users = User.from_union([User.where(id: 1), User.where(id: 2)])
#
# This would produce the following SQL query:
#
# SELECT *
# FROM (
# SELECT *
# FROM users
# WHERE id = 1
#
# UNION
#
# SELECT *
# FROM users
# WHERE id = 2
# ) users;
#
# members - An Array of ActiveRecord::Relation objects to use in the UNION.
#
# remove_duplicates - A boolean indicating if duplicate entries should be
# removed. Defaults to true.
#
# alias_as - The alias to use for the sub query. Defaults to the name of the
# table of the current model.
# rubocop: disable Gitlab/Union
def from_union(members, remove_duplicates: true, alias_as: table_name)
union = Gitlab::SQL::Union
.new(members, remove_duplicates: remove_duplicates)
.to_sql
# This pattern is necessary as a bug in Rails 4 can cause the use of
# `from("string here").includes(:foo)` to break ActiveRecord. This is
# fixed in https://github.com/rails/rails/pull/25374, which is released as
# part of Rails 5.
from([Arel.sql("(#{union}) #{alias_as}")])
end
# rubocop: enable Gitlab/Union
end
end
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