Avoid extra `show variables` in migration

`initialize_schema_migrations_table` is called in every migrations.

https://github.com/rails/rails/blob/v5.0.0.beta1/activerecord/lib/active_record/migration.rb#L1080
https://github.com/rails/rails/blob/v5.0.0.beta1/activerecord/lib/active_record/schema.rb#L51

This means that extra `show variables` is called regardless of the
existence of `schema_migrations` table.

This change is to avoid extra `show variables` if `schema_migrations`
table exists.

2 files changed, 10 insertions, 8 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
index 8db7f9172f..cc245587c1 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -115,7 +115,7 @@ module ActiveRecord
         checks = []
         checks << lambda { |c| c.name == column_name }
         checks << lambda { |c| c.type == type } if type
-        [:limit, :precision, :scale, :default, :null].each do |attr|
+        (migration_keys - [:name]).each do |attr|
           checks << lambda { |c| c.send(attr) == options[attr] } if options.key?(attr)
         end
 
@@ -981,6 +981,10 @@ module ActiveRecord
         ActiveRecord::InternalMetadata.create_table
       end
 
+      def internal_string_options_for_primary_key # :nodoc:
+        { primary_key: true }
+      end
+
       def assume_migrated_upto_version(version, migrations_paths)
         migrations_paths = Array(migrations_paths)
         version = version.to_i
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index 4dc7aa0c22..70d7956baa 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -67,14 +67,12 @@ module ActiveRecord
         end
       end
 
-      MAX_INDEX_LENGTH_FOR_CHARSETS_OF_4BYTES_MAXLEN = 191
       CHARSETS_OF_4BYTES_MAXLEN = ['utf8mb4', 'utf16', 'utf16le', 'utf32']
-      def initialize_schema_migrations_table
-        if CHARSETS_OF_4BYTES_MAXLEN.include?(charset)
-          ActiveRecord::SchemaMigration.create_table(MAX_INDEX_LENGTH_FOR_CHARSETS_OF_4BYTES_MAXLEN)
-        else
-          ActiveRecord::SchemaMigration.create_table
-        end
+
+      def internal_string_options_for_primary_key # :nodoc:
+        super.tap { |options|
+          options[:collation] = collation.sub(/\A[^_]+/, 'utf8') if CHARSETS_OF_4BYTES_MAXLEN.include?(charset)
+        }
       end
 
       def version