Fixed db:prepare task for multiple databases.

When one database existed already, but not the other,
during setup of missing one, existing database was wiped out.

activerecord/lib/active_record/railties/databases.rake

@@ -266,12 +266,31 @@ db_namespace = namespace :db do
 
   desc "Runs setup if database does not exist, or runs migrations if it does"
   task prepare: :load_config do
+    seed = false
+
     ActiveRecord::Base.configurations.configs_for(env_name: ActiveRecord::Tasks::DatabaseTasks.env).each do |db_config|
       ActiveRecord::Base.establish_connection(db_config.config)
-      db_namespace["migrate"].invoke
+
+      ActiveRecord::Tasks::DatabaseTasks.migrate
+
+      # Skipped when no database
+      ActiveRecord::Tasks::DatabaseTasks.dump_schema(db_config.config, ActiveRecord::Base.schema_format, db_config.spec_name)
+
     rescue ActiveRecord::NoDatabaseError
-      db_namespace["setup"].invoke
+      ActiveRecord::Tasks::DatabaseTasks.create_current(db_config.env_name, db_config.spec_name)
+      ActiveRecord::Tasks::DatabaseTasks.load_schema(
+        db_config.config,
+        ActiveRecord::Base.schema_format,
+        nil,
+        db_config.env_name,
+        db_config.spec_name
+      )
+
+      seed = true
     end
+
+    ActiveRecord::Base.establish_connection
+    ActiveRecord::Tasks::DatabaseTasks.load_seed if seed
   end
 
   desc "Loads the seed data from db/seeds.rb"
@@ -336,13 +355,9 @@ db_namespace = namespace :db do
   namespace :schema do
     desc "Creates a db/schema.rb file that is portable against any DB supported by Active Record"
     task dump: :load_config do
-      require "active_record/schema_dumper"
       ActiveRecord::Base.configurations.configs_for(env_name: ActiveRecord::Tasks::DatabaseTasks.env).each do |db_config|
-        filename = ActiveRecord::Tasks::DatabaseTasks.dump_filename(db_config.spec_name, :ruby)
-        File.open(filename, "w:utf-8") do |file|
-          ActiveRecord::Base.establish_connection(db_config.config)
-          ActiveRecord::SchemaDumper.dump(ActiveRecord::Base.connection, file)
-        end
+        ActiveRecord::Base.establish_connection(db_config.config)
+        ActiveRecord::Tasks::DatabaseTasks.dump_schema(db_config.config, :ruby, db_config.spec_name)
       end
 
       db_namespace["schema:dump"].reenable
@@ -385,14 +400,7 @@ db_namespace = namespace :db do
     task dump: :load_config do
       ActiveRecord::Base.configurations.configs_for(env_name: ActiveRecord::Tasks::DatabaseTasks.env).each do |db_config|
         ActiveRecord::Base.establish_connection(db_config.config)
-        filename = ActiveRecord::Tasks::DatabaseTasks.dump_filename(db_config.spec_name, :sql)
-        ActiveRecord::Tasks::DatabaseTasks.structure_dump(db_config.config, filename)
-        if ActiveRecord::SchemaMigration.table_exists?
-          File.open(filename, "a") do |f|
-            f.puts ActiveRecord::Base.connection.dump_schema_information
-            f.print "\n"
-          end
-        end
+        ActiveRecord::Tasks::DatabaseTasks.dump_schema(db_config.config, :sql, db_config.spec_name)
       end
 
       db_namespace["structure:dump"].reenable

activerecord/lib/active_record/tasks/database_tasks.rb

@@ -169,8 +169,8 @@ module ActiveRecord
         end
       end
 
-      def create_current(environment = env)
-        each_current_configuration(environment) { |configuration|
+      def create_current(environment = env, spec_name = nil)
+        each_current_configuration(environment, spec_name) { |configuration|
           create configuration
         }
         ActiveRecord::Base.establish_connection(environment.to_sym)
@@ -325,6 +325,26 @@ module ActiveRecord
         Migration.verbose = verbose_was
       end
 
+      def dump_schema(configuration, format = ActiveRecord::Base.schema_format, spec_name = "primary")
+        require "active_record/schema_dumper"
+        filename = dump_filename(spec_name, format)
+
+        case format
+        when :ruby
+          File.open(filename, "w:utf-8") do |file|
+            ActiveRecord::SchemaDumper.dump(ActiveRecord::Base.connection, file)
+          end
+        when :sql
+          structure_dump(configuration, filename)
+          if ActiveRecord::SchemaMigration.table_exists?
+            File.open(filename, "a") do |f|
+              f.puts ActiveRecord::Base.connection.dump_schema_information
+              f.print "\n"
+            end
+          end
+        end
+      end
+
       def schema_file(format = ActiveRecord::Base.schema_format)
         File.join(db_dir, schema_file_type(format))
       end
@@ -406,12 +426,14 @@ module ActiveRecord
           task.is_a?(String) ? task.constantize : task
         end
 
-        def each_current_configuration(environment)
+        def each_current_configuration(environment, spec_name = nil)
           environments = [environment]
           environments << "test" if environment == "development"
 
           environments.each do |env|
             ActiveRecord::Base.configurations.configs_for(env_name: env).each do |db_config|
+              next if spec_name && spec_name != db_config.spec_name
+
               yield db_config.config, db_config.spec_name, env
             end
           end

railties/test/application/rake/multi_dbs_test.rb

@@ -303,6 +303,26 @@ module ApplicationTests
         require "#{app_path}/config/environment"
         db_prepare
       end
+
+      test "db:prepare setups missing database without clearing existing one" do
+        require "#{app_path}/config/environment"
+        Dir.chdir(app_path) do
+          # Bug not visible on SQLite3. Can be simplified when https://github.com/rails/rails/issues/36383 resolved
+          use_postgresql(multi_db: true)
+          generate_models_for_animals
+
+          rails "db:create:animals", "db:migrate:animals", "db:create:primary", "db:migrate:primary", "db:schema:dump"
+          rails "db:drop:primary"
+          Dog.create!
+          output = rails("db:prepare")
+
+          assert_match(/Created database/, output)
+          assert_equal 1, Dog.count
+        ensure
+          Dog.connection.disconnect!
+          rails "db:drop" rescue nil
+        end
+      end
     end
   end
 end

railties/test/isolation/abstract_unit.rb

@@ -448,18 +448,36 @@ module TestHelpers
       $:.reject! { |path| path =~ %r'/(#{to_remove.join('|')})/' }
     end
 
-    def use_postgresql
-      File.open("#{app_path}/config/database.yml", "w") do |f|
-        f.puts <<-YAML
-        default: &default
-          adapter: postgresql
-          pool: 5
-          database: railties_test
-        development:
-          <<: *default
-        test:
-          <<: *default
-        YAML
+    def use_postgresql(multi_db: false)
+      if multi_db
+        File.open("#{app_path}/config/database.yml", "w") do |f|
+          f.puts <<-YAML
+          default: &default
+            adapter: postgresql
+            pool: 5
+          development:
+            primary:
+              <<: *default
+              database: railties_test
+            animals:
+              <<: *default
+              database: railties_animals_test
+              migrations_paths: db/animals_migrate
+          YAML
+        end
+      else
+        File.open("#{app_path}/config/database.yml", "w") do |f|
+          f.puts <<-YAML
+          default: &default
+            adapter: postgresql
+            pool: 5
+            database: railties_test
+          development:
+            <<: *default
+          test:
+            <<: *default
+          YAML
+        end
       end
     end
   end