Fix cyclic dependencies in operation graph for field.references (#949)

Original [PR](#948).
Renamed the branch from "issue#904" -> "issue-904" to prevent errors for
`pip install`.
Also fixed backward compatibility issues with graph constructions for
Croissant metadata using version 0.8 and 1.0, where `field.references`
can lead to `FileObject` or `FileSet` (similar to `field.source`).

Author: Neo3333

python/mlcroissant/mlcroissant/_src/structure_graph/graph.py

@@ -26,6 +26,7 @@
 from mlcroissant._src.core import constants
 from mlcroissant._src.core.types import Json
 from mlcroissant._src.structure_graph.base_node import Node
+from mlcroissant._src.structure_graph.nodes.field import Field
 from mlcroissant._src.structure_graph.nodes.file_object import FileObject
 from mlcroissant._src.structure_graph.nodes.file_set import FileSet
 from mlcroissant._src.structure_graph.nodes.record_set import RecordSet
@@ -63,13 +64,35 @@ def from_nodes_to_graph(metadata) -> nx.MultiDiGraph:
     for record_set in metadata.record_sets:
         for field in record_set.fields:
             _add_edge(graph, uuid_to_node, record_set.uuid, field)
-            for origin in [field.source, field.references]:
-                if origin:
-                    _add_edge(graph, uuid_to_node, origin.uuid, record_set)
+            if field.source:
+                _add_edge(graph, uuid_to_node, field.source.uuid, record_set)
+            if field.references:
+                referenced_node = uuid_to_node.get(field.references.uuid)
+                # If the referenced node is a "Field"
+                if isinstance(referenced_node, Field):
+                    # Dependency on references: Referenced Field -> Referencing Field
+                    _add_edge(graph, uuid_to_node, field.references.uuid, field)
+                # Backward compatible with earlier version of the Croissant spec
+                else:
+                    # If it is referencing other types of nodes such as FileObject
+                    _add_edge(graph, uuid_to_node, field.references.uuid, record_set)
             for sub_field in field.sub_fields:
-                for origin in [sub_field.source, sub_field.references]:
-                    if origin:
-                        _add_edge(graph, uuid_to_node, origin.uuid, record_set)
+                if sub_field.source:
+                    _add_edge(graph, uuid_to_node, sub_field.source.uuid, record_set)
+                if sub_field.references:
+                    referenced_node = uuid_to_node.get(field.references.uuid)
+                    # If the referenced node is a "Field"
+                    if isinstance(referenced_node, Field):
+                        # Dependency on references: Referenced Field -> Referencing Field
+                        _add_edge(
+                            graph, uuid_to_node, sub_field.references.uuid, sub_field
+                        )
+                    else:
+                        # If it is referencing other types of nodes such as FileObject
+                        _add_edge(
+                            graph, uuid_to_node, sub_field.references.uuid, record_set
+                        )
+
     # `Metadata` are used as the entry node.
     _add_node_as_entry_node(graph, metadata)
     return graph