test: cover parallel foreign-key edges to the same parent (#1492) (#1514)

Follow-up to #1508. The MultiDiGraph migration enables two foreign keys from one
child to the same parent as distinct parallel edges; the prior name-keyed
dependency dict collapsed them into one. LocalSynapse (two renamed FKs to Cell)
had no direct test for this. Asserts Cell appears as two parent edges of
LocalSynapse — across Table.parents()/children(), the dependency MultiDiGraph,
and the rendered Diagram — with distinct rename maps and edge keys.

Author: dimitri-yatsenko

tests/integration/test_foreign_keys.py

@@ -1,3 +1,4 @@
+import datajoint as dj
 from datajoint.declare import declare
 
 from tests.schema_advanced import (
@@ -51,3 +52,40 @@ def test_delete(schema_adv):
     to_delete = len(parent & "11 in (person_id, parent)")
     (person & "person_id=11").delete()
     assert to_delete and len(parent) == original_len - to_delete
+
+
+def test_parallel_edges_to_same_parent(schema_adv):
+    """Two foreign keys from one child to the same parent are preserved as two
+    distinct parallel edges (``nx.MultiDiGraph`` migration, #1492 / PR #1508).
+
+    ``LocalSynapse`` references ``Cell`` twice —
+    ``-> Cell.proj(presynaptic='cell')`` and ``-> Cell.proj(postsynaptic='cell')``
+    — so ``Cell`` must appear as TWO parent edges of ``LocalSynapse``. The
+    pre-migration name-keyed dependency dict collapsed these into a single entry;
+    this test locks the parallel-edges capability across all three layers that
+    consume it: the ``Dependencies`` accessors, the dependency graph itself, and
+    the rendered ``Diagram``.
+    """
+    cell_name = Cell.full_table_name
+    local_name = LocalSynapse.full_table_name
+
+    # (a) Table.parents() surfaces BOTH edges to Cell, with distinct rename maps.
+    cell_edges = [props for name, props in LocalSynapse().parents(foreign_key_info=True) if name == cell_name]
+    assert len(cell_edges) == 2, "LocalSynapse must expose two parallel FK edges to Cell"
+    renamed = {child for props in cell_edges for child, parent in props["attr_map"].items() if parent == "cell"}
+    assert renamed == {"presynaptic", "postsynaptic"}
+    # the two edges must carry distinct keys (``tuple(attr_map)``) — the uniqueness
+    # the keyed-edge migration relies on to keep them apart.
+    assert len({tuple(sorted(props["attr_map"].items())) for props in cell_edges}) == 2
+
+    # (b) The dependency graph keeps both edges — cascade/trace traverse per edge.
+    deps = LocalSynapse.connection.dependencies
+    deps.load(force=False)
+    assert deps.number_of_edges(cell_name, local_name) == 2
+    # symmetric view from the parent side (children direction)
+    local_as_child = [name for name, _ in Cell().children(foreign_key_info=True) if name == local_name]
+    assert len(local_as_child) == 2
+
+    # (c) The rendered Diagram (a MultiDiGraph over full table names) draws both.
+    diagram = dj.Diagram(schema_adv)
+    assert diagram.number_of_edges(cell_name, local_name) == 2