Detect mutations of arrays and array members

3 files changed, 41 insertions, 12 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index fc726d6519..823597fc92 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Detect in-place modifications of PG array types
+
+    *Sean Griffin*
+
 *   Add `bin/rake db:purge` task to empty the current database.
 
     *Yves Senn*
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/oid/array.rb b/activerecord/lib/active_record/connection_adapters/postgresql/oid/array.rb
index 63f3be45de..d322c56acc 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/oid/array.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/oid/array.rb
@@ -3,6 +3,20 @@ module ActiveRecord
     module PostgreSQL
       module OID # :nodoc:
         class Array < Type::Value
+          include Type::Mutable
+
+          # Loads pg_array_parser if available. String parsing can be
+          # performed quicker by a native extension, which will not create
+          # a large amount of Ruby objects that will need to be garbage
+          # collected. pg_array_parser has a C and Java extension
+          begin
+            require 'pg_array_parser'
+            include PgArrayParser
+          rescue LoadError
+            require 'active_record/connection_adapters/postgresql/array_parser'
+            include PostgreSQL::ArrayParser
+          end
+
           attr_reader :subtype, :delimiter
           delegate :type, to: :subtype
 
@@ -31,18 +45,6 @@ module ActiveRecord
             end
           end
 
-          # Loads pg_array_parser if available. String parsing can be
-          # performed quicker by a native extension, which will not create
-          # a large amount of Ruby objects that will need to be garbage
-          # collected. pg_array_parser has a C and Java extension
-          begin
-            require 'pg_array_parser'
-            include PgArrayParser
-          rescue LoadError
-            require 'active_record/connection_adapters/postgresql/array_parser'
-            include PostgreSQL::ArrayParser
-          end
-
           private
 
           def type_cast_array(value, method)
diff --git a/activerecord/test/cases/adapters/postgresql/array_test.rb b/activerecord/test/cases/adapters/postgresql/array_test.rb
index 15901b389e..266592e2c7 100644
--- a/activerecord/test/cases/adapters/postgresql/array_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/array_test.rb
@@ -16,6 +16,7 @@ class PostgresqlArrayTest < ActiveRecord::TestCase
         t.string 'tags', array: true
         t.integer 'ratings', array: true
         t.datetime :datetimes, array: true
+        t.hstore :hstores, array: true
       end
     end
     @column = PgArray.columns_hash['tags']
@@ -221,6 +222,28 @@ class PostgresqlArrayTest < ActiveRecord::TestCase
     assert_equal %({hello,;"world;"}), semicolon_delim.type_cast_for_database(strings)
   end
 
+  def test_mutate_array
+    x = PgArray.create!(tags: %w(one two))
+
+    x.tags << "three"
+    x.save!
+    x.reload
+
+    assert_equal %w(one two three), x.tags
+    assert_not x.changed?
+  end
+
+  def test_mutate_value_in_array
+    x = PgArray.create!(hstores: [{ a: 'a' }, { b: 'b' }])
+
+    x.hstores.first['a'] = 'c'
+    x.save!
+    x.reload
+
+    assert_equal [{ 'a' => 'c' }, { 'b' => 'b' }], x.hstores
+    assert_not x.changed?
+  end
+
   def test_datetime_with_timezone_awareness
     tz = "Pacific Time (US & Canada)"