Ensure Active Record connection consistency

Currently Active Record can be configured via the environment variable `DATABASE_URL` or by manually injecting a hash of values which is what Rails does, reading in `database.yml` and setting Active Record appropriately. Active Record expects to be able to use `DATABASE_URL` without the use of Rails, and we cannot rip out this functionality without deprecating. This presents a problem though when both config is set, and a `DATABASE_URL` is present. Currently the `DATABASE_URL` should "win" and none of the values in `database.yml` are used. This is somewhat unexpected to me if I were to set values such as `pool` in the `production:` group of `database.yml` they are ignored.

There are many ways that active record initiates a connection today:

- Stand Alone (without rails)
  - `rake db:<tasks>`
  - ActiveRecord.establish_connection
 
- With Rails
  - `rake db:<tasks>`
  - `rails <server> | <console>`
  - `rails dbconsole`


We should make all of these behave exactly the same way. The best way to do this is to put all of this logic in one place so it is guaranteed to be used.

Here is my prosed matrix of how this behavior should work:

```
No database.yml
No DATABASE_URL
=> Error
```

```
database.yml present
No DATABASE_URL
=> Use database.yml configuration
```

```
No database.yml
DATABASE_URL present
=> use DATABASE_URL configuration
```

```
database.yml present
DATABASE_URL present
=> Merged into `url` sub key. If both specify `url` sub key, the `database.yml` `url`
   sub key "wins". If other paramaters `adapter` or `database` are specified in YAML,
   they are discarded as the `url` sub key "wins".
```

### Implementation

Current implementation uses `ActiveRecord::Base.configurations` to resolve and merge all connection information before returning. This is achieved through a utility class: `ActiveRecord::ConnectionHandling::MergeAndResolveDefaultUrlConfig`.

To understand the exact behavior of this class, it is best to review the behavior in activerecord/test/cases/connection_adapters/connection_handler_test.rb though it should match the above proposal.

5 files changed, 80 insertions, 22 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/connection_specification.rb b/activerecord/lib/active_record/connection_adapters/connection_specification.rb
index 9f210c5f33..3f8b14bf67 100644
--- a/activerecord/lib/active_record/connection_adapters/connection_specification.rb
+++ b/activerecord/lib/active_record/connection_adapters/connection_specification.rb
@@ -123,13 +123,22 @@ module ActiveRecord
         def resolve(config)
           if config
             resolve_connection config
-          elsif defined?(Rails.env)
-            resolve_env_connection Rails.env.to_sym
+          elsif env = ActiveRecord::ConnectionHandling::RAILS_ENV.call
+            resolve_env_connection env.to_sym
           else
             raise AdapterNotSpecified
           end
         end
 
+        # Expands each key in @configurations hash into fully resolved hash
+        def resolve_all
+          config = configurations.dup
+          config.each do |key, value|
+            config[key] = resolve(value) if value
+          end
+          config
+        end
+
         # Returns an instance of ConnectionSpecification for a given adapter.
         # Accepts a hash one layer deep that contains all connection information.
         #
@@ -219,7 +228,7 @@ module ActiveRecord
           elsif spec.is_a?(String)
             resolve_string_connection(spec)
           else
-            raise(AdapterNotSpecified, "#{spec} database is not configured")
+            raise(AdapterNotSpecified, "'#{spec}' database is not configured. Available configuration: #{configurations.inspect}")
           end
         end
 
diff --git a/activerecord/lib/active_record/connection_handling.rb b/activerecord/lib/active_record/connection_handling.rb
index c4afadbd9b..11f6a47158 100644
--- a/activerecord/lib/active_record/connection_handling.rb
+++ b/activerecord/lib/active_record/connection_handling.rb
@@ -1,5 +1,8 @@
 module ActiveRecord
   module ConnectionHandling
+    RAILS_ENV   = -> { Rails.env if defined?(Rails) }
+    DEFAULT_ENV = -> { RAILS_ENV.call || "default_env" }
+
     # Establishes the connection to the database. Accepts a hash as input where
     # the <tt>:adapter</tt> key must be specified with the name of a database adapter (in lower-case)
     # example for regular databases (MySQL, Postgresql, etc):
@@ -41,9 +44,10 @@ module ActiveRecord
     #
     # The exceptions AdapterNotSpecified, AdapterNotFound and ArgumentError
     # may be returned on an error.
-    def establish_connection(spec = ENV["DATABASE_URL"])
-      resolver = ConnectionAdapters::ConnectionSpecification::Resolver.new configurations
-      spec = resolver.spec(spec)
+    def establish_connection(spec = nil)
+      spec     ||= DEFAULT_ENV.call.to_sym
+      resolver =   ConnectionAdapters::ConnectionSpecification::Resolver.new configurations
+      spec     =   resolver.spec(spec)
 
       unless respond_to?(spec.adapter_method)
         raise AdapterNotFound, "database configuration specifies nonexistent #{spec.config[:adapter]} adapter"
@@ -53,6 +57,56 @@ module ActiveRecord
       connection_handler.establish_connection self, spec
     end
 
+    class MergeAndResolveDefaultUrlConfig # :nodoc:
+      def initialize(raw_configurations, url = ENV['DATABASE_URL'])
+        @raw_config = raw_configurations.dup
+        @url        = url
+      end
+
+      # Returns fully resolved connection hashes.
+      # Merges connection information from `ENV['DATABASE_URL']` if available.
+      def resolve
+        ConnectionAdapters::ConnectionSpecification::Resolver.new(config).resolve_all
+      end
+
+      private
+        def config
+          if @url
+            raw_merged_into_default
+          else
+            @raw_config
+          end
+        end
+
+        def raw_merged_into_default
+          default = default_url_hash
+
+          @raw_config.each do |env, values|
+            default[env] = values || {}
+            default[env].merge!("url" => @url) { |h, v1, v2| v1 || v2 } if default[env].is_a?(Hash)
+          end
+          default
+        end
+
+        # When the raw configuration is not present and ENV['DATABASE_URL']
+        # is available we return a hash with the connection information in
+        # the connection URL. This hash responds to any string key with
+        # resolved connection information.
+        def default_url_hash
+          if @raw_config.blank?
+            Hash.new do |hash, key|
+              hash[key] = if key.is_a? String
+                 ActiveRecord::ConnectionAdapters::ConnectionSpecification::ConnectionUrlResolver.new(@url).to_hash
+              else
+                nil
+              end
+            end
+          else
+            {}
+          end
+        end
+    end
+
     # Returns the connection currently associated with the class. This can
     # also be used to "borrow" the connection to do database work unrelated
     # to any of the specific Active Records.
diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index 18ee77f6fe..cd8690d500 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -42,9 +42,16 @@ module ActiveRecord
       #         'database' => 'db/production.sqlite3'
       #      }
       #   }
-      mattr_accessor :configurations, instance_writer: false
+      def self.configurations=(config)
+        @@configurations = ActiveRecord::ConnectionHandling::MergeAndResolveDefaultUrlConfig.new(config).resolve
+      end
       self.configurations = {}
 
+      # Returns fully resolved configurations hash
+      def self.configurations
+        @@configurations
+      end
+
       ##
       # :singleton-method:
       # Determines whether to use Time.utc (using :utc) or Time.local (using :local) when pulling
diff --git a/activerecord/lib/active_record/railtie.rb b/activerecord/lib/active_record/railtie.rb
index ec85b3c843..11b564f8f9 100644
--- a/activerecord/lib/active_record/railtie.rb
+++ b/activerecord/lib/active_record/railtie.rb
@@ -40,19 +40,7 @@ module ActiveRecord
 
       namespace :db do
         task :load_config do
-          configuration = if ENV["DATABASE_URL"]
-            { Rails.env => ENV["DATABASE_URL"] }
-          else
-            Rails.application.config.database_configuration || {}
-          end
-
-          resolver = ActiveRecord::ConnectionAdapters::ConnectionSpecification::Resolver.new(configuration)
-
-          configuration.each do |key, value|
-            configuration[key] = resolver.resolve(value) if value
-          end
-
-          ActiveRecord::Tasks::DatabaseTasks.database_configuration = configuration
+          ActiveRecord::Tasks::DatabaseTasks.database_configuration = Rails.application.config.database_configuration
 
           if defined?(ENGINE_PATH) && engine = Rails::Engine.find(ENGINE_PATH)
             if engine.paths['db/migrate'].existent
@@ -137,7 +125,7 @@ module ActiveRecord
           end
         end
 
-        self.configurations = app.config.database_configuration || {}
+        self.configurations = Rails.application.config.database_configuration
         establish_connection
       end
     end
diff --git a/activerecord/lib/active_record/railties/databases.rake b/activerecord/lib/active_record/railties/databases.rake
index 58dfa2c5a5..561387a179 100644
--- a/activerecord/lib/active_record/railties/databases.rake
+++ b/activerecord/lib/active_record/railties/databases.rake
@@ -2,7 +2,7 @@ require 'active_record'
 
 db_namespace = namespace :db do
   task :load_config do
-    ActiveRecord::Base.configurations = ActiveRecord::Tasks::DatabaseTasks.database_configuration || {}
+    ActiveRecord::Base.configurations       = ActiveRecord::Tasks::DatabaseTasks.database_configuration || {}
     ActiveRecord::Migrator.migrations_paths = ActiveRecord::Tasks::DatabaseTasks.migrations_paths
   end