Refactor graph result to support edges

Author: Tobias Hafner

org/polypheny/prism/graph_frame.proto

@@ -18,14 +18,24 @@ package org.polypheny.prism;
  * The GraphFrame represents the non-relational result of a query in the labeled property graph model.
  * If the query returns a relational result, such as when aggregations are used or specific properties are returned,
  * a relational frame is used. The GraphFrame is used for results where a set of nodes and/or edges is returned.
+ * The nodes or edges are wrapped in graph elements to allow them to be mixed as paths are returned as a mixed collection of both.
  * If a result produces multiple frames (used to transmit large results), all results will be of the same type (relational vs. graph).
- * If the result contains paths, these are represented as a list of nodes and a list of edges. Thus both lists can be populated at once.
  */
 message GraphFrame {
-  // Field used to represent a set of nodes as part of a result. If this field is populated all others must not be set.
-  repeated ProtoNode nodes = 1;
-  // Field used to represent a set of edges as part of a result. If this field is populated all others must not be set.
-  repeated ProtoEdge edges = 2;
+  repeated GraphElement element = 1;
+}
+
+/*
+ * Represents an element in a graph. Each element is either a node or an edge.
+ */
+message GraphElement {
+  // This field stores the graph element itself which is either a node or an edge.
+  oneof element {
+    // This field stores the node if the graph element is a node.
+    ProtoNode node = 1;
+    // This field stores the edge if the graph element is an edge.
+    ProtoEdge edge = 2;
+  }
 }