Allow separate database env variables per-connection

This commit adds a feature which allows separate database ENV variables
to be defined for each spec in a 3-tier config. The names for the
environment variables will be `#{name.upcase}_DATABASE_URL`

This commit also introduces a change in behavior around handling of
`DATABASE_URL`. Instead of using `DATABASE_URL` to change _all_ specs
in a multi-database configuration, it will now only affect the `primary`
connection.

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Make the DATABASE_URL env variable only affect the primary connection. Add new env variables for multiple databases.
+
+    *John Crepezzi*, *Eileen Uchitelle*
+
 *   Add a warning for enum elements with 'not_' prefix.
 
         class Foo
@@ -56,5 +60,4 @@
 
     *Michael Duchemin*
 
-
 Please check [6-0-stable](https://github.com/rails/rails/blob/6-0-stable/activerecord/CHANGELOG.md) for previous changes.

activerecord/lib/active_record/database_configurations.rb

@@ -168,12 +168,19 @@ module ActiveRecord
       end
 
       def environment_url_config(env, spec_name, config)
-        url = ENV["DATABASE_URL"]
+        url = environment_value_for(spec_name)
         return unless url
 
         ActiveRecord::DatabaseConfigurations::UrlConfig.new(env, spec_name, url, config)
       end
 
+      def environment_value_for(spec_name)
+        spec_env_key = "#{spec_name.upcase}_DATABASE_URL"
+        url = ENV[spec_env_key]
+        url ||= ENV["DATABASE_URL"] if spec_name == "primary"
+        url
+      end
+
       def method_missing(method, *args, &blk)
         case method
         when :each, :first

activerecord/test/cases/connection_adapters/merge_and_resolve_default_url_config_test.rb

@@ -334,6 +334,8 @@ module ActiveRecord
           }
         }
 
+        configs = ActiveRecord::DatabaseConfigurations.new(config)
+        actual = configs.configs_for(env_name: "default_env", spec_name: "primary").config
         expected = {
           "adapter"  => "postgresql",
           "database" => "foo",
@@ -341,11 +343,37 @@ module ActiveRecord
           "pool"     => 5
         }
 
-        ["primary", "animals"].each do |spec_name|
-          configs = ActiveRecord::DatabaseConfigurations.new(config)
-          actual = configs.configs_for(env_name: "default_env", spec_name: spec_name).config
-          assert_equal expected, actual
-        end
+        assert_equal expected, actual
+
+        configs = ActiveRecord::DatabaseConfigurations.new(config)
+        actual = configs.configs_for(env_name: "default_env", spec_name: "animals").config
+        expected = { "pool" => 5 }
+
+        assert_equal expected, actual
+      end
+
+      def test_separate_database_env_vars
+        ENV["DATABASE_URL"] = "postgres://localhost/foo"
+        ENV["PRIMARY_DATABASE_URL"] = "postgres://localhost/primary"
+        ENV["ANIMALS_DATABASE_URL"] = "postgres://localhost/animals"
+
+        config = {
+          "default_env" => {
+            "primary" => { "pool" => 5 },
+            "animals" => { "pool" => 5 }
+          }
+        }
+
+        configs = ActiveRecord::DatabaseConfigurations.new(config)
+        actual = configs.configs_for(env_name: "default_env", spec_name: "primary").config
+        assert_equal "primary", actual["database"]
+
+        configs = ActiveRecord::DatabaseConfigurations.new(config)
+        actual = configs.configs_for(env_name: "default_env", spec_name: "animals").config
+        assert_equal "animals", actual["database"]
+      ensure
+        ENV.delete("PRIMARY_DATABASE_URL")
+        ENV.delete("ANIMALS_DATABASE_URL")
       end
 
       def test_does_not_change_other_environments