Fix exists usage

src/Model/Table/Table.php

@@ -12,6 +12,7 @@
 use Cake\Validation\Validator;
 use Exception;
 use InvalidArgumentException;
+use RuntimeException;
 
 class Table extends CoreTable {
 
@@ -343,12 +344,17 @@ public function fieldByConditions($name, array $conditions = [], array $customOp
 	/**
 	 * 2.x shim for exists() and primary key.
 	 *
+	 * @deprecated Not usable as array, only for single primary keys. Use exists() directly.
 	 * @param int $id
 	 * @return bool
 	 */
 	public function existsById($id) {
+		$primaryKey = $this->primaryKey();
+		if (is_array($primaryKey)) {
+			throw new RuntimeException('Not supported with multiple primary keys');
+		}
 		$conditions = [
-			$this->primaryKey() => $id
+			$primaryKey => $id
 		];
 		return parent::exists($conditions);
 	}