Support relation.and for intersection as Set theory

As described at #39328, `relation.merge` behaves inspired as Hash-like
merge for where clause. In other words, currently there is no official
way to intersect the result by both relation conditions (i.e. there is
no official way to maintain both relation conditions).

To resolve that issue, I'd like to support a way to intersect relations
as `relation.and`.

```ruby
david_and_mary = Author.where(id: [david, mary])
mary_and_bob   = Author.where(id: [mary, bob]) # => [bob]

david_and_mary.merge(mary_and_bob) # => [mary, bob]

david_and_mary.and(mary_and_bob) # => [mary]
david_and_mary.or(mary_and_bob)  # => [david, mary, bob]
```

Fixes #39232.

activerecord/CHANGELOG.md

@@ -1,3 +1,17 @@
+*   Support `relation.and` for intersection as Set theory.
+
+    ```ruby
+    david_and_mary = Author.where(id: [david, mary])
+    mary_and_bob   = Author.where(id: [mary, bob]) # => [bob]
+
+    david_and_mary.merge(mary_and_bob) # => [mary, bob]
+
+    david_and_mary.and(mary_and_bob) # => [mary]
+    david_and_mary.or(mary_and_bob)  # => [david, mary, bob]
+    ```
+
+    *Ryuta Kamizono*
+
 *   Merging conditions on the same column no longer maintain both conditions,
     and will be consistently replaced by the latter condition in Rails 6.2.
     To migrate to Rails 6.2's behavior, use `relation.merge(other, rewhere: true)`.

activerecord/lib/active_record/querying.rb

@@ -13,9 +13,10 @@ module Querying
       :destroy_all, :delete_all, :update_all, :touch_all, :destroy_by, :delete_by,
       :find_each, :find_in_batches, :in_batches,
       :select, :reselect, :order, :reorder, :group, :limit, :offset, :joins, :left_joins, :left_outer_joins,
-      :where, :rewhere, :preload, :extract_associated, :eager_load, :includes, :from, :lock, :readonly, :extending, :or,
-      :having, :create_with, :distinct, :references, :none, :unscope, :optimizer_hints, :merge, :except, :only,
-      :count, :average, :minimum, :maximum, :sum, :calculate, :annotate,
+      :where, :rewhere, :preload, :extract_associated, :eager_load, :includes, :from, :lock, :readonly,
+      :and, :or, :annotate, :optimizer_hints, :extending,
+      :having, :create_with, :distinct, :references, :none, :unscope, :merge, :except, :only,
+      :count, :average, :minimum, :maximum, :sum, :calculate,
       :pluck, :pick, :ids, :strict_loading
     ].freeze # :nodoc:
     delegate(*QUERYING_METHODS, to: :all)

activerecord/lib/active_record/relation/query_methods.rb

@@ -699,6 +699,38 @@ def rewhere(conditions)
       scope
     end
 
+    # Returns a new relation, which is the logical intersection of this relation and the one passed
+    # as an argument.
+    #
+    # The two relations must be structurally compatible: they must be scoping the same model, and
+    # they must differ only by #where (if no #group has been defined) or #having (if a #group is
+    # present).
+    #
+    #    Post.where(id: [1, 2]).and(Post.where(id: [2, 3]))
+    #    # SELECT `posts`.* FROM `posts` WHERE `posts`.`id` IN (1, 2) AND `posts`.`id` IN (2, 3)
+    #
+    def and(other)
+      if other.is_a?(Relation)
+        spawn.and!(other)
+      else
+        raise ArgumentError, "You have passed #{other.class.name} object to #and. Pass an ActiveRecord::Relation object instead."
+      end
+    end
+
+    def and!(other) # :nodoc:
+      incompatible_values = structurally_incompatible_values_for_or(other)
+
+      unless incompatible_values.empty?
+        raise ArgumentError, "Relation passed to #and must be structurally compatible. Incompatible values: #{incompatible_values}"
+      end
+
+      self.where_clause |= other.where_clause
+      self.having_clause |= other.having_clause
+      self.references_values |= other.references_values
+
+      self
+    end
+
     # Returns a new relation, which is the logical union of this relation and the one passed as an
     # argument.
     #
@@ -710,11 +742,11 @@ def rewhere(conditions)
     #    # SELECT `posts`.* FROM `posts` WHERE ((id = 1) OR (author_id = 3))
     #
     def or(other)
-      unless other.is_a? Relation
+      if other.is_a?(Relation)
+        spawn.or!(other)
+      else
         raise ArgumentError, "You have passed #{other.class.name} object to #or. Pass an ActiveRecord::Relation object instead."
       end
-
-      spawn.or!(other)
     end
 
     def or!(other) # :nodoc:

activerecord/lib/active_record/relation/where_clause.rb

@@ -19,6 +19,10 @@ def -(other)
         WhereClause.new(predicates - other.predicates)
       end
 
+      def |(other)
+        WhereClause.new(predicates | other.predicates)
+      end
+
       def merge(other, rewhere = nil)
         predicates = if rewhere
           except_predicates(other.extract_attributes)

activerecord/test/cases/relation/merging_test.rb

@@ -14,7 +14,7 @@ class RelationMergingTest < ActiveRecord::TestCase
   fixtures :developers, :comments, :authors, :author_addresses, :posts
 
   def test_merge_in_clause
-    david, mary, bob = authors(:david, :mary, :bob)
+    david, mary, bob = authors = authors(:david, :mary, :bob)
 
     david_and_mary = Author.where(id: [david, mary]).order(:id)
     mary_and_bob   = Author.where(id: [mary, bob]).order(:id)
@@ -35,18 +35,24 @@ def test_merge_in_clause
 
     assert_equal [mary, bob], david_and_mary.merge(mary_and_bob)
     assert_equal [mary, bob], david_and_mary.merge(mary_and_bob, rewhere: true)
+    assert_equal [mary], david_and_mary.and(mary_and_bob)
+    assert_equal authors, david_and_mary.or(mary_and_bob)
 
     assert_equal [david, mary], mary_and_bob.merge(david_and_mary)
     assert_equal [david, mary], mary_and_bob.merge(david_and_mary, rewhere: true)
+    assert_equal [mary], david_and_mary.and(mary_and_bob)
+    assert_equal authors, david_and_mary.or(mary_and_bob)
 
-    david_and_bob = Author.where(id: david).or(Author.where(name: "Bob"))
+    david_and_bob = Author.where(id: david).or(Author.where(name: "Bob")).order(:id)
 
     assert_equal [david], david_and_mary.merge(david_and_bob)
     assert_equal [david], david_and_mary.merge(david_and_bob, rewhere: true)
+    assert_equal [david], david_and_mary.and(david_and_bob)
+    assert_equal authors, david_and_mary.or(david_and_bob)
   end
 
   def test_merge_between_clause
-    david, mary, bob = authors(:david, :mary, :bob)
+    david, mary, bob = authors = authors(:david, :mary, :bob)
 
     david_and_mary = Author.where(id: david.id..mary.id).order(:id)
     mary_and_bob   = Author.where(id: mary.id..bob.id).order(:id)
@@ -80,20 +86,26 @@ def test_merge_between_clause
       assert_equal [mary], david_and_mary.merge(mary_and_bob)
     end
     assert_equal [mary, bob], david_and_mary.merge(mary_and_bob, rewhere: true)
+    assert_equal [mary], david_and_mary.and(mary_and_bob)
+    assert_equal authors, david_and_mary.or(mary_and_bob)
 
     assert_deprecated(message) do
       assert_equal [mary], mary_and_bob.merge(david_and_mary)
     end
     assert_equal [david, mary], mary_and_bob.merge(david_and_mary, rewhere: true)
+    assert_equal [mary], david_and_mary.and(mary_and_bob)
+    assert_equal authors, david_and_mary.or(mary_and_bob)
 
-    david_and_bob = Author.where(id: david).or(Author.where(name: "Bob"))
+    david_and_bob = Author.where(id: david).or(Author.where(name: "Bob")).order(:id)
 
     assert_equal [david], david_and_mary.merge(david_and_bob)
     assert_equal [david], david_and_mary.merge(david_and_bob, rewhere: true)
+    assert_equal [david], david_and_mary.and(david_and_bob)
+    assert_equal authors, david_and_mary.or(david_and_bob)
   end
 
   def test_merge_or_clause
-    david, mary, bob = authors(:david, :mary, :bob)
+    david, mary, bob = authors = authors(:david, :mary, :bob)
 
     david_and_mary = Author.where(id: david).or(Author.where(id: mary)).order(:id)
     mary_and_bob   = Author.where(id: mary).or(Author.where(id: bob)).order(:id)
@@ -127,16 +139,22 @@ def test_merge_or_clause
       assert_equal [mary], david_and_mary.merge(mary_and_bob)
     end
     assert_equal [mary, bob], david_and_mary.merge(mary_and_bob, rewhere: true)
+    assert_equal [mary], david_and_mary.and(mary_and_bob)
+    assert_equal authors, david_and_mary.or(mary_and_bob)
 
     assert_deprecated(message) do
       assert_equal [mary], mary_and_bob.merge(david_and_mary)
     end
     assert_equal [david, mary], mary_and_bob.merge(david_and_mary, rewhere: true)
+    assert_equal [mary], david_and_mary.and(mary_and_bob)
+    assert_equal authors, david_and_mary.or(mary_and_bob)
 
-    david_and_bob = Author.where(id: david).or(Author.where(name: "Bob"))
+    david_and_bob = Author.where(id: david).or(Author.where(name: "Bob")).order(:id)
 
     assert_equal [david], david_and_mary.merge(david_and_bob)
     assert_equal [david], david_and_mary.merge(david_and_bob, rewhere: true)
+    assert_equal [david], david_and_mary.and(david_and_bob)
+    assert_equal authors, david_and_mary.or(david_and_bob)
   end
 
   def test_merge_not_in_clause