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.

1 files changed, 127 insertions, 0 deletions

diff --git a/activerecord/test/cases/connection_adapters/connection_handler_test.rb b/activerecord/test/cases/connection_adapters/connection_handler_test.rb
index 3e33b30144..3365ad1294 100644
--- a/activerecord/test/cases/connection_adapters/connection_handler_test.rb
+++ b/activerecord/test/cases/connection_adapters/connection_handler_test.rb
@@ -2,6 +2,133 @@ require "cases/helper"
 
 module ActiveRecord
   module ConnectionAdapters
+
+    class MergeAndResolveDefaultUrlConfigTest < ActiveRecord::TestCase
+
+      def klass
+        ActiveRecord::ConnectionHandling::MergeAndResolveDefaultUrlConfig
+      end
+
+      def setup
+        @previous_database_url = ENV.delete("DATABASE_URL")
+      end
+
+      def teardown
+        ENV["DATABASE_URL"] = @previous_database_url if @previous_database_url
+      end
+
+      def test_string_connection
+        config   = { "production" => "postgres://localhost/foo" }
+        actual   = klass.new(config).resolve
+        expected = { "production" =>
+                     { "adapter"  => "postgresql",
+                       "database" => "foo",
+                       "host"     => "localhost"
+                      }
+                    }
+        assert_equal expected, actual
+      end
+
+      def test_url_sub_key
+        config   = { "production" => { "url" => "postgres://localhost/foo" } }
+        actual   = klass.new(config).resolve
+        expected = { "production" =>
+                     { "adapter"  => "postgresql",
+                       "database" => "foo",
+                       "host"     => "localhost"
+                      }
+                    }
+        assert_equal expected, actual
+      end
+
+      def test_hash
+        config = { "production" => { "adapter" => "postgres", "database" => "foo" } }
+        actual = klass.new(config).resolve
+        assert_equal config, actual
+      end
+
+      def test_blank
+        config = {}
+        actual = klass.new(config).resolve
+        assert_equal config, actual
+      end
+
+      def test_blank_with_database_url
+        ENV['DATABASE_URL'] = "postgres://localhost/foo"
+
+        config   = {}
+        actual   = klass.new(config).resolve
+        expected = { "adapter"  => "postgresql",
+                     "database" => "foo",
+                     "host"     => "localhost" }
+        assert_equal expected, actual["production"]
+        assert_equal expected, actual["development"]
+        assert_equal expected, actual["test"]
+        assert_equal nil,      actual[:production]
+        assert_equal nil,      actual[:development]
+        assert_equal nil,      actual[:test]
+      end
+
+      def test_sting_with_database_url
+        ENV['DATABASE_URL'] = "NOT-POSTGRES://localhost/NOT_FOO"
+
+        config   = { "production" => "postgres://localhost/foo" }
+        actual   = klass.new(config).resolve
+
+        expected = { "production" =>
+                     { "adapter"  => "postgresql",
+                       "database" => "foo",
+                       "host"     => "localhost"
+                      }
+                    }
+        assert_equal expected, actual
+      end
+
+      def test_url_sub_key_with_database_url
+        ENV['DATABASE_URL'] = "NOT-POSTGRES://localhost/NOT_FOO"
+
+        config   = { "production" => { "url" => "postgres://localhost/foo" } }
+        actual   = klass.new(config).resolve
+        expected = { "production" =>
+                    { "adapter"  => "postgresql",
+                       "database" => "foo",
+                       "host"     => "localhost"
+                      }
+                    }
+        assert_equal expected, actual
+      end
+
+      def test_merge_no_conflicts_with_database_url
+        ENV['DATABASE_URL'] = "postgres://localhost/foo"
+
+        config   = {"production" => { "pool" => "5" } }
+        actual   = klass.new(config).resolve
+        expected = { "production" =>
+                     { "adapter"  => "postgresql",
+                       "database" => "foo",
+                       "host"     => "localhost",
+                       "pool"     => "5"
+                      }
+                    }
+        assert_equal expected, actual
+      end
+
+      def test_merge_conflicts_with_database_url
+        ENV['DATABASE_URL'] = "postgres://localhost/foo"
+
+        config   = {"production" => { "adapter" => "NOT-POSTGRES", "database" => "NOT-FOO", "pool" => "5" } }
+        actual   = klass.new(config).resolve
+        expected = { "production" =>
+                     { "adapter"  => "postgresql",
+                       "database" => "foo",
+                       "host"     => "localhost",
+                       "pool"     => "5"
+                      }
+                    }
+        assert_equal expected, actual
+      end
+    end
+
     class ConnectionHandlerTest < ActiveRecord::TestCase
       def setup
         @klass    = Class.new(Base)   { def self.name; 'klass';    end }