Restore legacy YAML compatibility for MySQL with `active_record_yaml_…

…version: 1`

We had removed the dedicated `MysqlDateTime`, `MysqlJson`, and
`OID::Json` classes in the past (f1a0fa9, #29666), so legacy YAML
loading has no longer always perfectly compatiblity.

Fortunately, v2 (Rails 5.1 style) YAML doesn't contain type information
in almost all cases (unless serializing object using custom select), so
usually removing dedicated type affects to legacy YAML older than v1
(Rails 5.0 style) YAML only.

This restores legacy YAML compatibility for MySQL with v1 format by
adding the class alias in YAML for `MysqlString` which is most recently
reported about compatibility concern. It also affects to legacy Rails
4.2 style YAML, but 4.2 style YAML had already broken by removing
`MysqlDateTime` over 4 years ago.

activerecord/lib/active_record.rb

@@ -186,3 +186,4 @@ def self.eager_load!
 YAML.load_tags["!ruby/object:ActiveRecord::AttributeSet"] = "ActiveModel::AttributeSet"
 YAML.load_tags["!ruby/object:ActiveRecord::Attribute::FromDatabase"] = "ActiveModel::Attribute::FromDatabase"
 YAML.load_tags["!ruby/object:ActiveRecord::LazyAttributeHash"] = "ActiveModel::LazyAttributeHash"
+YAML.load_tags["!ruby/object:ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter::MysqlString"] = "ActiveRecord::Type::String"

activerecord/test/cases/yaml_serialization_test.rb

@@ -93,6 +93,24 @@ def test_a_yaml_version_is_provided_for_future_backwards_compat
     assert coder["active_record_yaml_version"]
   end
 
+  def test_deserializing_rails_v2_yaml
+    topic = YAML.load(yaml_fixture("rails_v2"))
+
+    assert_not_predicate topic, :new_record?
+    assert_equal 1, topic.id
+    assert_equal "The First Topic", topic.title
+    assert_equal "Have a nice day", topic.content
+  end
+
+  def test_deserializing_rails_v1_mysql_yaml
+    topic = YAML.load(yaml_fixture("rails_v1_mysql"))
+
+    assert_not_predicate topic, :new_record?
+    assert_equal 1, topic.id
+    assert_equal "The First Topic", topic.title
+    assert_equal "Have a nice day", topic.content
+  end
+
   def test_deserializing_rails_41_yaml
     topic = assert_deprecated do
       YAML.load(yaml_fixture("rails_4_1"))

activerecord/test/support/yaml_compatibility_fixtures/rails_v1_mysql.yml

@@ -0,0 +1,207 @@
+--- !ruby/object:Topic
+raw_attributes:
+  id: 1
+  title: The First Topic
+  author_name: David
+  author_email_address: david@loudthinking.com
+  written_on: &5 2003-07-16 14:28:11.223300000 Z
+  bonus_time: &6 2000-01-01 14:28:00.000000000 Z
+  last_read: 2004-04-15
+  content: |
+    --- Have a nice day
+    ...
+  important: 
+  approved: 0
+  replies_count: 1
+  unique_replies_count: 0
+  parent_id: 
+  parent_title: 
+  type: 
+  group: 
+  created_at: &7 2020-06-25 10:25:48.335322000 Z
+  updated_at: &8 2020-06-25 10:25:48.335322000 Z
+attributes: !ruby/object:ActiveRecord::AttributeSet
+  attributes: !ruby/object:ActiveRecord::LazyAttributeHash
+    types:
+      id: &9 !ruby/object:ActiveRecord::Type::Integer
+        precision: 
+        scale: 
+        limit: 
+        range: !ruby/range
+          begin: -2147483648
+          end: 2147483648
+          excl: true
+      title: &10 !ruby/object:ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter::MysqlString
+        precision: 
+        scale: 
+        limit: 250
+      author_name: &1 !ruby/object:ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter::MysqlString
+        precision: 
+        scale: 
+        limit: 255
+      author_email_address: *1
+      written_on: &4 !ruby/object:ActiveRecord::Type::DateTime
+        precision: 6
+        scale: 
+        limit: 
+      bonus_time: &11 !ruby/object:ActiveRecord::Type::Time
+        precision: 0
+        scale: 
+        limit: 
+      last_read: &12 !ruby/object:ActiveRecord::Type::Date
+        precision: 
+        scale: 
+        limit: 
+      content: &13 !ruby/object:ActiveRecord::Type::Serialized
+        subtype: &2 !ruby/object:ActiveRecord::Type::Text
+          precision: 
+          scale: 
+          limit: 65535
+        coder: !ruby/object:ActiveRecord::Coders::YAMLColumn
+          attr_name: :content
+          object_class: !ruby/class 'Object'
+        delegate_dc_obj: *2
+      important: *2
+      approved: &14 !ruby/object:ActiveRecord::Type::Boolean
+        precision: 
+        scale: 
+        limit: 
+      replies_count: &3 !ruby/object:ActiveRecord::Type::Integer
+        precision: 
+        scale: 
+        limit: 
+        range: !ruby/range
+          begin: -2147483648
+          end: 2147483648
+          excl: true
+      unique_replies_count: *3
+      parent_id: *3
+      parent_title: *1
+      type: *1
+      group: *1
+      created_at: *4
+      updated_at: *4
+    values:
+      id: 1
+      title: The First Topic
+      author_name: David
+      author_email_address: david@loudthinking.com
+      written_on: *5
+      bonus_time: *6
+      last_read: 2004-04-15
+      content: |
+        --- Have a nice day
+        ...
+      important: 
+      approved: 0
+      replies_count: 1
+      unique_replies_count: 0
+      parent_id: 
+      parent_title: 
+      type: 
+      group: 
+      created_at: *7
+      updated_at: *8
+    additional_types: {}
+    materialized: true
+    delegate_hash:
+      id: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: id
+        value_before_type_cast: 1
+        type: *9
+        original_attribute: 
+      title: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: title
+        value_before_type_cast: The First Topic
+        type: *10
+        original_attribute: 
+      author_name: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: author_name
+        value_before_type_cast: David
+        type: *1
+        original_attribute: 
+      author_email_address: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: author_email_address
+        value_before_type_cast: david@loudthinking.com
+        type: *1
+        original_attribute: 
+      written_on: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: written_on
+        value_before_type_cast: *5
+        type: *4
+        original_attribute: 
+      bonus_time: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: bonus_time
+        value_before_type_cast: *6
+        type: *11
+        original_attribute: 
+      last_read: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: last_read
+        value_before_type_cast: 2004-04-15
+        type: *12
+        original_attribute: 
+      content: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: content
+        value_before_type_cast: |
+          --- Have a nice day
+          ...
+        type: *13
+        original_attribute: 
+      important: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: important
+        value_before_type_cast: 
+        type: *2
+        original_attribute: 
+      approved: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: approved
+        value_before_type_cast: 0
+        type: *14
+        original_attribute: 
+      replies_count: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: replies_count
+        value_before_type_cast: 1
+        type: *3
+        original_attribute: 
+      unique_replies_count: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: unique_replies_count
+        value_before_type_cast: 0
+        type: *3
+        original_attribute: 
+      parent_id: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: parent_id
+        value_before_type_cast: 
+        type: *3
+        original_attribute: 
+      parent_title: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: parent_title
+        value_before_type_cast: 
+        type: *1
+        original_attribute: 
+      type: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: type
+        value_before_type_cast: 
+        type: *1
+        original_attribute: 
+      group: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: group
+        value_before_type_cast: 
+        type: *1
+        original_attribute: 
+      created_at: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: created_at
+        value_before_type_cast: *7
+        type: *4
+        original_attribute: 
+      updated_at: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: updated_at
+        value_before_type_cast: *8
+        type: *4
+        original_attribute: 
+    default_attributes:
+      id: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: id
+        value_before_type_cast: 
+        type: *9
+        original_attribute: 
+new_record: false
+active_record_yaml_version: 1

activerecord/test/support/yaml_compatibility_fixtures/rails_v2.yml

@@ -0,0 +1,55 @@
+--- !ruby/object:Topic
+concise_attributes:
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: id
+  value_before_type_cast: 1
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: title
+  value_before_type_cast: The First Topic
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: author_name
+  value_before_type_cast: David
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: author_email_address
+  value_before_type_cast: david@loudthinking.com
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: written_on
+  value_before_type_cast: '2003-07-16 14:28:11.223300'
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: bonus_time
+  value_before_type_cast: '2000-01-01 14:28:00'
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: last_read
+  value_before_type_cast: '2004-04-15'
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: content
+  value_before_type_cast: |
+    --- Have a nice day
+    ...
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: important
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: approved
+  value_before_type_cast: 0
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: replies_count
+  value_before_type_cast: 1
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: unique_replies_count
+  value_before_type_cast: 0
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: parent_id
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: parent_title
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: type
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: group
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: created_at
+  value_before_type_cast: '2020-06-25 09:52:17.503713'
+- !ruby/object:ActiveModel::Attribute::FromDatabase
+  name: updated_at
+  value_before_type_cast: '2020-06-25 09:52:17.503713'
+new_record: false
+active_record_yaml_version: 2