_id is free

common/proto/table_definition.proto

@@ -200,6 +200,17 @@ message TableDefinitionResponse {
 
 // Describes the tree of all profiles and their tables.
 message ProfileTreeResponse {
+  // One link: the table it points at, and the column carrying it.
+  message Dependency {
+    // Table being referenced.
+    string table_name = 1;
+
+    // Column holding the reference, named by whoever declared the link. This
+    // is what identifies the link, since a table may point at one target
+    // several times.
+    string column_name = 2;
+  }
+
   // Table entry in a profile.
   message Table {
     // Internal ID from table_definitions.id (metadata record).
@@ -208,8 +219,9 @@ message ProfileTreeResponse {
     // Table name within the profile (schema).
     string name = 2;
 
-    // Other tables this one references (based on link definitions only).
-    repeated string depends_on = 3;
+    // Links this table declares. One entry per link, so a table that names the
+    // same target twice appears twice.
+    repeated Dependency depends_on = 3;
 
     // Columns whose values make up the human-readable row label, in order.
     repeated string row_display_columns = 4;