Add db:prepare rake task.

activerecord/lib/active_record/railties/databases.rake

@@ -222,6 +222,16 @@ db_namespace = namespace :db do
   desc "Creates the database, loads the schema, and initializes with the seed data (use db:reset to also drop the database first)"
   task setup: ["db:schema:load_if_ruby", "db:structure:load_if_sql", :seed]
 
+  desc "Runs setup if database does not exist, or runs migrations if it does"
+  task prepare: :load_config do
+    ActiveRecord::Base.configurations.configs_for(env_name: Rails.env).each do |db_config|
+      ActiveRecord::Base.establish_connection(db_config.config)
+      db_namespace["migrate"].invoke
+    rescue ActiveRecord::NoDatabaseError
+      db_namespace["setup"].invoke
+    end
+  end
+
   desc "Loads the seed data from db/seeds.rb"
   task seed: :load_config do
     db_namespace["abort_if_pending_migrations"].invoke

railties/test/application/rake/dbs_test.rb

@@ -553,6 +553,15 @@ def db_test_load_structure
           end
         end
       end
+
+      test "db:prepare setup the database" do
+        Dir.chdir(app_path) do
+          rails "generate", "model", "book", "title:string"
+          output = rails("db:prepare")
+
+          assert_match(/CreateBooks: migrated/, output)
+        end
+      end
     end
   end
 end

railties/test/application/rake/multi_dbs_test.rb

@@ -137,6 +137,21 @@ def db_migrate_status_namespaced(namespace)
         end
       end
 
+      def db_prepare
+        Dir.chdir(app_path) do
+          generate_models_for_animals
+          output = rails("db:prepare")
+
+          ActiveRecord::Base.configurations.configs_for(env_name: Rails.env).each do |db_config|
+            if db_config.spec_name == "primary"
+              assert_match(/CreateBooks: migrated/, output)
+            else
+              assert_match(/CreateDogs: migrated/, output)
+            end
+          end
+        end
+      end
+
       def write_models_for_animals
         # make a directory for the animals migration
         FileUtils.mkdir_p("#{app_path}/db/animals_migrate")
@@ -226,6 +241,11 @@ def generate_models_for_animals
         require "#{app_path}/config/environment"
         db_migrate_and_schema_cache_dump_and_schema_cache_clear
       end
+
+      test "db:prepare works on all databases" do
+        require "#{app_path}/config/environment"
+        db_prepare
+      end
     end
   end
 end