Add ActiveRecord::Relation#structurally_compatible?.

activerecord/CHANGELOG.md

@@ -1,3 +1,11 @@
+*   Add `ActiveRecord::Relation#structurally_compatible?`.
+
+    Adds a query method by which a user can tell if the relation that they're
+    about to use for `#or` or `#and` is structurally compatible with the
+    receiver.
+
+    *Kevin Newton*
+
 *   Add `ActiveRecord::QueryMethods#in_order_of`.
 
     This allows you to specify an explicit order that you'd like records

activerecord/lib/active_record/relation/query_methods.rb

@@ -748,6 +748,21 @@ def invert_where! # :nodoc:
       self
     end
 
+    # Checks whether the given relation is structurally compatible with this relation, to determine
+    # if it's possible to use the #and and #or methods without raising an error. Structurally
+    # compatible is defined as: 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").structurally_compatible?(Post.where("author_id = 3"))
+    #    # => true
+    #
+    #    Post.joins(:comments).structurally_compatible?(Post.where("id = 1"))
+    #    # => false
+    #
+    def structurally_compatible?(other)
+      structurally_incompatible_values_for(other).empty?
+    end
+
     # Returns a new relation, which is the logical intersection of this relation and the one passed
     # as an argument.
     #

activerecord/test/cases/relation/delegation_test.rb

@@ -50,7 +50,7 @@ class QueryingMethodsDelegationTest < ActiveRecord::TestCase
       ActiveRecord::FinderMethods.public_instance_methods(false) - [:include?, :member?, :raise_record_not_found_exception!] +
       ActiveRecord::SpawnMethods.public_instance_methods(false) - [:spawn, :merge!] +
       ActiveRecord::QueryMethods.public_instance_methods(false).reject { |method|
-        method.end_with?("=", "!", "value", "values", "clause")
+        method.end_with?("=", "!", "?", "value", "values", "clause")
       } - [:reverse_order, :arel, :extensions, :construct_join_dependency] + [
         :any?, :many?, :none?, :one?,
         :first_or_create, :first_or_create!, :first_or_initialize,

activerecord/test/cases/relation/structural_compatibility_test.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "models/post"
+
+module ActiveRecord
+  class StructuralCompatibilityTest < ActiveRecord::TestCase
+    fixtures :posts
+
+    def test_compatible_values
+      left = Post.where(id: 1)
+      right = Post.where(id: 2)
+
+      assert left.structurally_compatible?(right)
+    end
+
+    def test_incompatible_single_value_relations
+      left = Post.distinct.where("id = 1")
+      right = Post.where(id: [2, 3])
+
+      assert_not left.structurally_compatible?(right)
+    end
+
+    def test_incompatible_multi_value_relations
+      left = Post.order("body asc").where("id = 1")
+      right = Post.order("id desc").where(id: [2, 3])
+
+      assert_not left.structurally_compatible?(right)
+    end
+
+    def test_incompatible_unscope
+      left = Post.order("body asc").where("id = 1").unscope(:order)
+      right = Post.order("body asc").where("id = 2")
+
+      assert_not left.structurally_compatible?(right)
+    end
+  end
+end