Update/fix how db connections are retrieved and released (#802)

spec/avram/queryable_spec.cr

@@ -62,6 +62,12 @@ describe Avram::Queryable do
     query.statement.should eq "SELECT #{User::COLUMN_SQL} FROM users WHERE users.age >= $1 AND users.name = $2"
   end
 
+  it "releases connection if no open transaction", tags: Avram::SpecHelper::TRUNCATE do
+    UserQuery.new.first?
+
+    TestDatabase.connections.should be_empty
+  end
+
   describe "#distinct" do
     it "selects distinct" do
       query = UserQuery.new.distinct.query

src/avram/database.cr

@@ -153,7 +153,9 @@ abstract class Avram::Database
 
   # :nodoc:
   def run
-    yield current_connection || db
+    with_connection do |conn|
+      yield conn
+    end
   end
 
   # :nodoc:
@@ -173,16 +175,31 @@ abstract class Avram::Database
     end
   end
 
-  private def current_connection : DB::Connection
-    connections[object_id] ||= db.checkout
+  # singular place to retrieve a DB::Connection
+  # must be passed a block and we
+  # try to release the connection back to the pool
+  # once the block is finished
+  private def with_connection
+    key = object_id
+    connections[key] ||= db.checkout
+    connection = connections[key]
+
+    begin
+      yield connection
+    ensure
+      if !connection._avram_in_transaction?
+        connection.release
+        connections.delete(key)
+      end
+    end
   end
 
   private def object_id : UInt64
     self.class.lock_id || Fiber.current.object_id
   end
 
-  private def current_transaction : DB::Transaction?
-    current_connection._avram_stack.last?
+  private def current_transaction(connection : DB::Connection) : DB::Transaction?
+    connection._avram_stack.last?
   end
 
   protected def truncate
@@ -199,12 +216,14 @@ abstract class Avram::Database
 
   # :nodoc:
   def transaction : Bool
-    if current_transaction.try(&._avram_joinable?)
-      yield
-      true
-    else
-      wrap_in_transaction do
+    with_connection do |conn|
+      if current_transaction(conn).try(&._avram_joinable?)
         yield
+        true
+      else
+        wrap_in_transaction(conn) do
+          yield
+        end
       end
     end
   end
@@ -213,18 +232,13 @@ abstract class Avram::Database
     self.class.connections
   end
 
-  private def wrap_in_transaction
-    (current_transaction || current_connection).transaction do
+  private def wrap_in_transaction(conn)
+    (current_transaction(conn) || conn).transaction do
       yield
     end
     true
   rescue e : Avram::Rollback
     false
-  ensure
-    if !current_connection._avram_in_transaction?
-      current_connection.release
-      connections.delete(object_id)
-    end
   end
 
   class DatabaseCleaner