Fix extract default with CURRENT_TIMESTUMP

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -495,12 +495,16 @@ module ActiveRecord
       end
 
       # Returns an array of +Column+ objects for the table specified by +table_name+.
-      def columns(table_name)#:nodoc:
+      def columns(table_name) # :nodoc:
         sql = "SHOW FULL FIELDS FROM #{quote_table_name(table_name)}"
         execute_and_free(sql, 'SCHEMA') do |result|
           each_hash(result).map do |field|
             type_metadata = fetch_type_metadata(field[:Type], field[:Extra])
-            new_column(field[:Field], field[:Default], type_metadata, field[:Null] == "YES", nil, field[:Collation])
+            if type_metadata.type == :datetime && field[:Default] == "CURRENT_TIMESTAMP"
+              new_column(field[:Field], nil, type_metadata, field[:Null] == "YES", field[:Default], field[:Collation])
+            else
+              new_column(field[:Field], field[:Default], type_metadata, field[:Null] == "YES", nil, field[:Collation])
+            end
           end
         end
       end

activerecord/test/cases/defaults_test.rb

@@ -96,6 +96,17 @@ if current_adapter?(:PostgreSQLAdapter)
 end
 
 if current_adapter?(:Mysql2Adapter)
+  class MysqlDefaultExpressionTest < ActiveRecord::TestCase
+    include SchemaDumpingHelper
+
+    if ActiveRecord::Base.connection.version >= '5.6.0'
+      test "schema dump includes default expression" do
+        output = dump_table_schema("datetime_defaults")
+        assert_match %r/t\.datetime\s+"modified_datetime",\s+default: -> { "CURRENT_TIMESTAMP" }/, output
+      end
+    end
+  end
+
   class DefaultsTestWithoutTransactionalFixtures < ActiveRecord::TestCase
     # ActiveRecord::Base#create! (and #save and other related methods) will
     # open a new transaction. When in transactional tests mode, this will

activerecord/test/schema/mysql2_specific_schema.rb

@@ -1,4 +1,11 @@
 ActiveRecord::Schema.define do
+
+  if ActiveRecord::Base.connection.version >= '5.6.0'
+    create_table :datetime_defaults, force: true do |t|
+      t.datetime :modified_datetime, default: -> { 'CURRENT_TIMESTAMP' }
+    end
+  end
+
   create_table :binary_fields, force: true do |t|
     t.binary :var_binary, limit: 255
     t.binary :var_binary_large, limit: 4095