Fix exists() being called on every save() operation.

If an entity isNew and contains no primary key data we should not call
exists() as there is no way for it to exist yet. Exists should continue
to be called when the entity has a primary key value and is marked as
new though as it may or may not exist in the database and we need to
know.

Fixes #4260

src/ORM/Table.php

@@ -1148,7 +1148,7 @@ protected function _processSave($entity, $options) {
 		$primaryColumns = (array)$this->primaryKey();
 		$primary = $entity->extract($primaryColumns);
 
-		if ($primary && $entity->isNew()) {
+		if (array_filter($primary) !== [] && $entity->isNew()) {
 			$alias = $this->alias();
 			$conditions = [];
 			foreach ($primary as $k => $v) {

tests/TestCase/ORM/TableTest.php

@@ -1212,6 +1212,30 @@ public function testSaveNewEmptyEntity() {
 		$this->assertFalse($table->save($entity));
 	}
 
+/**
+ * Test that saving a new empty entity does not call exists.
+ *
+ * @group save
+ * @return void
+ */
+	public function testSaveNewEntityNoExists() {
+		$table = $this->getMock(
+			'Cake\ORM\Table',
+			['exists'],
+			[[
+				'connection' => $this->connection,
+				'alias' => 'Users',
+				'table' => 'users',
+			]]
+		);
+		$entity = $table->newEntity(['username' => 'mark']);
+		$this->assertTrue($entity->isNew());
+
+		$table->expects($this->never())
+			->method('exists');
+		$this->assertSame($entity, $table->save($entity));
+	}
+
 /**
  * Tests that saving an entity will filter out properties that
  * are not present in the table schema when saving