Allow bigint with default nil for avoiding auto increment primary key

Such as #10404, #18206.

activerecord/CHANGELOG.md

+*   Allow bigint with default nil for avoiding auto increment primary key.
+
+    *Ryuta Kamizono*
+
 *   Remove `DEFAULT_CHARSET` and `DEFAULT_COLLATION` in `MySQLDatabaseTasks`.
 
     We should omit the collation entirely rather than providing a default.

activerecord/lib/active_record/connection_adapters/mysql/schema_definitions.rb

@@ -3,7 +3,7 @@ module ConnectionAdapters
     module MySQL
       module ColumnMethods
         def primary_key(name, type = :primary_key, **options)
-          options[:auto_increment] = true if type == :bigint
+          options[:auto_increment] = true if type == :bigint && !options.key?(:default)
           super
         end
 

activerecord/lib/active_record/connection_adapters/mysql/schema_dumper.rb

@@ -4,8 +4,11 @@ module MySQL
       module ColumnDumper
         def column_spec_for_primary_key(column)
           spec = {}
-          if column.auto_increment?
-            spec[:id] = ':bigint' if column.bigint?
+          if column.bigint?
+            spec[:id] = ':bigint'
+            spec[:default] = schema_default(column) || 'nil' unless column.auto_increment?
+            spec[:unsigned] = 'true' if column.unsigned?
+          elsif column.auto_increment?
             spec[:unsigned] = 'true' if column.unsigned?
             return if spec.empty?
           else

activerecord/test/cases/primary_keys_test.rb

@@ -280,6 +280,32 @@ def test_primary_key_method_with_ansi_quotes
       con.reconnect!
     end
   end
+
+  class PrimaryKeyBigintNilDefaultTest < ActiveRecord::TestCase
+    include SchemaDumpingHelper
+
+    self.use_transactional_tests = false
+
+    def setup
+      @connection = ActiveRecord::Base.connection
+      @connection.create_table(:bigint_defaults, id: :bigint, default: nil, force: true)
+    end
+
+    def teardown
+      @connection.drop_table :bigint_defaults, if_exists: true
+    end
+
+    test "primary key with bigint allows default override via nil" do
+      column = @connection.columns(:bigint_defaults).find { |c| c.name == 'id' }
+      assert column.bigint?
+      assert_not column.auto_increment?
+    end
+
+    test "schema dump primary key with bigint default nil" do
+      schema = dump_table_schema "bigint_defaults"
+      assert_match %r{create_table "bigint_defaults", id: :bigint, default: nil}, schema
+    end
+  end
 end
 
 if current_adapter?(:PostgreSQLAdapter, :MysqlAdapter, :Mysql2Adapter)