Adding ability to listen for notifications from postgres

spec/database_spec.cr

@@ -0,0 +1,17 @@
+require "./spec_helper"
+
+describe Avram::Database do
+  describe "listen" do
+    it "yields the payload from a notify" do
+      done = Channel(Nil).new
+      TestDatabase.listen("dinner_time") do |notification|
+        notification.channel.should eq "dinner_time"
+        notification.payload.should eq "Tacos"
+        done.send(nil)
+      end
+
+      TestDatabase.exec("SELECT pg_notify('dinner_time', 'Tacos')")
+      done.receive
+    end
+  end
+end

src/avram/connection.cr

@@ -7,6 +7,12 @@ class Avram::Connection
     try_connection!
   end
 
+  def connect_listen(*channels : String, &block : PQ::Notification ->) : Nil
+    PG.connect_listen(@connection_string, *channels, &block)
+  rescue DB::ConnectionRefused
+    raise ConnectionError.new(connection_uri, database_class: @database_class)
+  end
+
   def try_connection!
     DB.open(@connection_string)
   rescue DB::ConnectionRefused

src/avram/database.cr

@@ -39,6 +39,20 @@ abstract class Avram::Database
     new.delete
   end
 
+  # Listens for `pg_notify()` calls on each channel in `channels`
+  # Yields a `PQ::Notification` object with `channel`, `payload`, and `pid`.
+  #
+  # ```
+  # # pg_notify("callback", "123")
+  # AppDatabase.listen("callback", "jobs") do |notification|
+  #   notification.channel # => "callback"
+  #   notification.payload # => "123"
+  # end
+  # ```
+  def self.listen(*channels : String, &block : PQ::Notification ->) : Nil
+    new.listen(*channels, &block)
+  end
+
   @@database_info : DatabaseInfo?
 
   def self.database_info : DatabaseInfo
@@ -131,11 +145,20 @@ abstract class Avram::Database
     yield current_transaction.try(&.connection) || db
   end
 
+  # :nodoc:
+  def listen(*channels : String, &block : PQ::Notification ->) : Nil
+    connection.connect_listen(*channels, &block)
+  end
+
+  private def connection : Avram::Connection
+    Avram::Connection.new(url, database_class: self.class)
+  end
+
   private def db : DB::Database
     @@db ||= @@lock.synchronize do
       # check @@db again because a previous request could have set it after
       # the first time it was checked
-      @@db || Avram::Connection.new(url, database_class: self.class).open
+      @@db || connection.open
     end
   end