Adding the ability to register a callback funciton to override the

results of Query::count()

src/ORM/Query.php

@@ -143,6 +143,14 @@ class Query extends DatabaseQuery {
  */
 	protected $_cache;
 
+/**
+ * A callable function that can be used to calculate the total amount of
+ * records this query will match when not using `limit`
+ *
+ * @var callable
+ */
+	protected $_counter;
+
 /**
  * Constuctor
  *
@@ -873,19 +881,46 @@ public function first() {
 	public function count() {
 		$query = clone $this;
 		$query->limit(null);
+		$query->offset(null);
+		$counter = $this->_counter;
+
+		if ($counter) {
+			$query->counter(null);
+			return (int)$counter($query);
+		}
 
 		// Forcing at least one field to be selected
 		$query->select($query->newExpr()->add('1'));
 		$statement = $this->connection()->newQuery()
 			->select(['count' => $query->func()->count('*')])
-			->from(['source' => $query])
+			->from(['count_source' => $query])
 			->execute();
 		$result = $statement->fetch('assoc')['count'];
 
 		$statement->closeCursor();
 		return (int)$result;
 	}
 
+/**
+ * Registers a callable function that will be executed when the `count` method in
+ * this query is called. The return value for the function will be set as the
+ * return value of the `count` method.
+ *
+ * This is particularly useful when you need to optimize a query for returning the
+ * count, for example removing unnecessary joins, removing group by or just return
+ * an estimated number of rows.
+ *
+ * The callback will receive as first argument a  clone of this query and not this
+ * query itself.
+ *
+ * @param callable $counter
+ * @return Cake\ORM\Query
+ */
+	public function counter($counter) {
+		$this->_counter = $counter;
+		return $this;
+	}
+
 /**
  * Toggle hydrating entites.
  *

tests/TestCase/ORM/QueryTest.php

@@ -1515,6 +1515,27 @@ public function testCountWithGroup() {
 		$this->assertEquals(2, $result);
 	}
 
+/**
+ * Tests that it is possible to provide
+ *
+ * @return void
+ */
+	public function testCountWuthCustomCounter() {
+		$table = TableRegistry::get('articles');
+		$query = $table->find('all');
+		$query
+			->select(['author_id', 's' => $query->func()->sum('id')])
+			->where(['id >' => 2])
+			->group(['author_id'])
+			->counter(function($q) use ($query) {
+				$this->assertNotSame($q, $query);
+				return $q->select([], true)->group([], true)->count();
+			});
+
+		$result = $query->count();
+		$this->assertEquals(1, $result);
+	}
+
 /**
  * Test update method.
  *