Tell how to Create a Database in Error Message

Currently if you attempt to use a database that does not exist  you get an error:

```
PG::ConnectionBad FATAL:  database "db_error" does not exist
```

The solution is easy, create and migrate your database however new developers may not know these commands by memory. Instead of requiring the developer to search for a solution, tell them how to fix the problem in the error message:

```
ActiveRecord::NoDatabase: FATAL:  database "db_error" does not exist
Run `$ bin/rake db:create db:migrate` to create your database
```

Active Record should not know about `rake db:migrate` so this additional information needs to come from the railtie. Potential alternative implementation suggestions are welcome.

3 files changed, 26 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
index dd3bfa5546..95cd69d5ad 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
@@ -865,6 +865,12 @@ module ActiveRecord
           PostgreSQLColumn.money_precision = (postgresql_version >= 80300) ? 19 : 10
 
           configure_connection
+        rescue ::PG::Error => error
+          if error.message.include?("does not exist")
+            raise ActiveRecord::NoDatabaseError.new(error.message)
+          else
+            raise error
+          end
         end
 
         # Configures the encoding, verbosity, schema search path, and time zone of the connection.
diff --git a/activerecord/lib/active_record/errors.rb b/activerecord/lib/active_record/errors.rb
index 2602f79db9..7f6228131f 100644
--- a/activerecord/lib/active_record/errors.rb
+++ b/activerecord/lib/active_record/errors.rb
@@ -94,6 +94,18 @@ module ActiveRecord
   class PreparedStatementInvalid < ActiveRecordError
   end
 
+  # Raised when a given database does not exist
+  class NoDatabaseError < ActiveRecordError
+    def initialize(message)
+      super extend_message(message)
+    end
+
+    # can be over written to add additional error information.
+    def extend_message(message)
+      message
+    end
+  end
+
   # Raised on attempt to save stale record. Record is stale when it's being saved in another query after
   # instantiation, for example, when two users edit the same wiki page and one starts editing and saves
   # the page before the other.
diff --git a/activerecord/lib/active_record/railtie.rb b/activerecord/lib/active_record/railtie.rb
index eef08aea88..ff98c829f4 100644
--- a/activerecord/lib/active_record/railtie.rb
+++ b/activerecord/lib/active_record/railtie.rb
@@ -122,6 +122,14 @@ module ActiveRecord
     # and then establishes the connection.
     initializer "active_record.initialize_database" do |app|
       ActiveSupport.on_load(:active_record) do
+
+        class ActiveRecord::NoDatabaseError
+          def extend_message(message)
+            message << "Run `$ bin/rake db:create db:migrate` to create your database"
+            message
+          end
+        end
+
         self.configurations = app.config.database_configuration || {}
         establish_connection
       end