[tracker] libtracker-data: Tell TrackerPredicateVariable to return graph if needed

[Carlos Garnacho <carlosg src gnome org>]

commit aa2c2dace5e2dad7737b594e12c2b86ba1dfcb9b
Author: Carlos Garnacho <carlosg gnome org>
Date:   Sat Nov 17 17:20:38 2018 +0100

    libtracker-data: Tell TrackerPredicateVariable to return graph if needed
    
    This got lost in the new parser, fixes a tracker-writeback query involving
    variables in graph and predicate. Pointed out by Sam Thursfield on
    https://gitlab.gnome.org/GNOME/tracker-miners/merge_requests/27#note_367289
    
    Enabling the return_graph variable, we create the appropriate column and
    the "no such column: predicate1.graph" error goes away.

 src/libtracker-data/tracker-sparql.c | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/src/libtracker-data/tracker-sparql.c b/src/libtracker-data/tracker-sparql.c
index 22ad078dd..ccbd2ca74 100644
--- a/src/libtracker-data/tracker-sparql.c
+++ b/src/libtracker-data/tracker-sparql.c
@@ -1078,6 +1078,9 @@ _add_quad (TrackerSparql  *sparql,
                tracker_binding_set_db_column_name (binding, "predicate");
                _add_binding (sparql, binding);
                g_object_unref (binding);
+
+               if (!tracker_token_is_empty (graph))
+                       pred_var->return_graph = TRUE;
        } else {
                /* The parser disallows parameter predicates */
                g_assert_not_reached ();