Add `ActiveRecord::Relation#readonly?`

Indicates whether a relation was marked readonly.

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Add `ActiveRecord::Relation.readonly?`
+
+    Reflects if the relation has been marked as readonly.
+
+    *Theodor Tonum*
+
 *   Improve `ActiveRecord::Store` to raise a descriptive exception if the column is not either
     structured (e.g., PostgreSQL +hstore+/+json+, or MySQL +json+) or declared serializable via
     `ActiveRecord.store`.

activerecord/lib/active_record/relation.rb

@@ -1261,6 +1261,10 @@ module ActiveRecord
       records.blank?
     end
 
+    def readonly?
+      readonly_value
+    end
+
     def values
       @values.dup
     end

activerecord/test/cases/readonly_test.rb

@@ -79,9 +79,11 @@ class ReadOnlyTest < ActiveRecord::TestCase
 
   def test_find_with_readonly_option
     Developer.all.each { |d| assert_not d.readonly? }
+    Developer.all.tap { |rel| assert_not rel.readonly? }
     Developer.readonly(false).each { |d| assert_not d.readonly? }
     Developer.readonly(true).each { |d| assert_predicate d, :readonly? }
     Developer.readonly.each { |d| assert_predicate d, :readonly? }
+    Developer.readonly.tap { |rel| assert_predicate rel, :readonly? }
   end
 
   def test_find_with_joins_option_does_not_imply_readonly