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.

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

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

activerecord/lib/active_record/internal_metadata.rb

@@ -5,10 +5,6 @@ module ActiveRecord
   # This class is used to create a table that keeps track of values and keys such
   # as which environment migrations were run in.
   class InternalMetadata < ActiveRecord::Base # :nodoc:
-    # Keys in mysql are limited to 191 characters, due to this no adapter can
-    # use a longer key
-    KEY_LIMIT = 191
-
     class << self
       def primary_key
         "key"
@@ -33,8 +29,10 @@ module ActiveRecord
       # Creates an internal metadata table with columns +key+ and +value+
       def create_table
         unless table_exists?
+          key_options = connection.internal_string_options_for_primary_key
+
           connection.create_table(table_name, id: false) do |t|
-            t.string :key, primary_key: true, limit: KEY_LIMIT
+            t.string :key, key_options
             t.string :value
             t.timestamps
           end

activerecord/lib/active_record/schema_migration.rb

@@ -20,10 +20,9 @@ module ActiveRecord
         ActiveSupport::Deprecation.silence { connection.table_exists?(table_name) }
       end
 
-      def create_table(limit=nil)
+      def create_table
         unless table_exists?
-          version_options = { primary_key: true }
-          version_options[:limit] = limit if limit
+          version_options = connection.internal_string_options_for_primary_key
 
           connection.create_table(table_name, id: false) do |t|
             t.string :version, version_options

activerecord/test/cases/adapters/mysql2/schema_migrations_test.rb

@@ -33,11 +33,6 @@ class SchemaMigrationsTest < ActiveRecord::Mysql2TestCase
     end
   end
 
-  def test_key_limit_max_matches_max
-    assert_equal ActiveRecord::InternalMetadata::KEY_LIMIT ,ActiveRecord::ConnectionAdapters::Mysql2Adapter::MAX_INDEX_LENGTH_FOR_CHARSETS_OF_4BYTES_MAXLEN
-  end
-
-
   private
 
   def with_encoding_utf8mb4