Adding legacy primary key should be compatible

Currently implicit legacy primary key is compatible, but adding explicit
legacy primary key is not compatible. It should also be fixed.

Fixes #30664.

activerecord/lib/active_record/migration/compatibility.rb

@@ -20,6 +20,11 @@ module ActiveRecord
 
       class V5_0 < V5_1
         module TableDefinition
+          def primary_key(name, type = :primary_key, **options)
+            type = :integer if type == :primary_key
+            super
+          end
+
           def references(*args, **options)
             super(*args, type: :integer, **options)
           end
@@ -86,6 +91,20 @@ module ActiveRecord
           end
         end
 
+        def add_column(table_name, column_name, type, options = {})
+          if type == :primary_key
+            case adapter_name
+            when "PostgreSQL"
+              type = :serial
+            when "Mysql2"
+              type = :integer
+              options[:auto_increment] = true
+            end
+            options[:primary_key] = true
+          end
+          super
+        end
+
         def add_reference(table_name, ref_name, **options)
           super(table_name, ref_name, type: :integer, **options)
         end

activerecord/test/cases/migration/compatibility_test.rb

@@ -199,6 +199,57 @@ class LegacyPrimaryKeyTest < ActiveRecord::TestCase
     assert_match %r{create_table "legacy_primary_keys", id: :integer, default: nil}, schema
   end
 
+  if current_adapter?(:Mysql2Adapter, :PostgreSQLAdapter)
+    def test_legacy_primary_key_in_create_table_should_be_integer
+      @migration = Class.new(ActiveRecord::Migration[5.0]) {
+        def change
+          create_table :legacy_primary_keys, id: false do |t|
+            t.primary_key :id
+          end
+        end
+      }.new
+
+      @migration.migrate(:up)
+
+      schema = dump_table_schema "legacy_primary_keys"
+      assert_match %r{create_table "legacy_primary_keys", id: :(?:integer|serial), (?!default: nil)}, schema
+    end
+
+    def test_legacy_primary_key_in_change_table_should_be_integer
+      @migration = Class.new(ActiveRecord::Migration[5.0]) {
+        def change
+          create_table :legacy_primary_keys, id: false do |t|
+            t.integer :dummy
+          end
+          change_table :legacy_primary_keys do |t|
+            t.primary_key :id
+          end
+        end
+      }.new
+
+      @migration.migrate(:up)
+
+      schema = dump_table_schema "legacy_primary_keys"
+      assert_match %r{create_table "legacy_primary_keys", id: :(?:integer|serial), (?!default: nil)}, schema
+    end
+
+    def test_add_column_with_legacy_primary_key_should_be_integer
+      @migration = Class.new(ActiveRecord::Migration[5.0]) {
+        def change
+          create_table :legacy_primary_keys, id: false do |t|
+            t.integer :dummy
+          end
+          add_column :legacy_primary_keys, :id, :primary_key
+        end
+      }.new
+
+      @migration.migrate(:up)
+
+      schema = dump_table_schema "legacy_primary_keys"
+      assert_match %r{create_table "legacy_primary_keys", id: :(?:integer|serial), (?!default: nil)}, schema
+    end
+  end
+
   def test_legacy_join_table_foreign_keys_should_be_integer
     @migration = Class.new(ActiveRecord::Migration[5.0]) {
       def change