Improve error message for #or when it is structurally incompatible

When you are using scopes and you chaining these scopes it is hard to
know which are the values that are incompatible. This way you can read
the message and know for which values you need to look for.

[Herminio Torres]

activerecord/lib/active_record/relation/query_methods.rb

@@ -659,8 +659,10 @@ module ActiveRecord
     end
 
     def or!(other) # :nodoc:
-      unless structurally_compatible_for_or?(other)
-        raise ArgumentError, 'Relation passed to #or must be structurally compatible'
+      incompatible_values = structurally_incompatible_values_for_or(other)
+
+      unless incompatible_values.empty?
+        raise ArgumentError, "Relation passed to #or must be structurally compatible. Incompatible values: #{incompatible_values}"
       end
 
       self.where_clause = self.where_clause.or(other.where_clause)
@@ -1189,10 +1191,10 @@ module ActiveRecord
       end
     end
 
-    def structurally_compatible_for_or?(other)
-      Relation::SINGLE_VALUE_METHODS.all? { |m| send("#{m}_value") == other.send("#{m}_value") } &&
-        (Relation::MULTI_VALUE_METHODS - [:extending]).all? { |m| send("#{m}_values") == other.send("#{m}_values") } &&
-        (Relation::CLAUSE_METHODS - [:having, :where]).all? { |m| send("#{m}_clause") == other.send("#{m}_clause") }
+    def structurally_incompatible_values_for_or(other)
+      Relation::SINGLE_VALUE_METHODS.reject { |m| send("#{m}_value") == other.send("#{m}_value") } +
+        (Relation::MULTI_VALUE_METHODS - [:extending]).reject { |m| send("#{m}_values") == other.send("#{m}_values") } +
+        (Relation::CLAUSE_METHODS - [:having, :where]).reject { |m| send("#{m}_clause") == other.send("#{m}_clause") }
     end
 
     def new_where_clause

activerecord/test/cases/relation/or_test.rb

@@ -52,9 +52,11 @@ module ActiveRecord
     end
 
     def test_or_with_incompatible_relations
-      assert_raises ArgumentError do
+      error = assert_raises ArgumentError do
         Post.order('body asc').where('id = 1').or(Post.order('id desc').where(:id => [2, 3])).to_a
       end
+
+      assert_equal "Relation passed to #or must be structurally compatible. Incompatible values: [:order]", error.message
     end
 
     def test_or_when_grouping