Allow GraphStore and Graph to have zero relationships

Co-authored-by: Florentin Dörre <florentin.dorre@neotechnology.com>

Author: Mats-SX

core/src/main/java/org/neo4j/gds/core/loading/CSRGraphStore.java

@@ -49,12 +49,10 @@
 import org.neo4j.gds.api.schema.RelationshipPropertySchema;
 import org.neo4j.gds.api.schema.RelationshipSchema;
 import org.neo4j.gds.core.Aggregation;
-import org.neo4j.gds.core.ProcedureConstants;
 import org.neo4j.gds.core.huge.CSRCompositeRelationshipIterator;
 import org.neo4j.gds.core.huge.HugeGraph;
 import org.neo4j.gds.core.huge.NodeFilteredGraph;
 import org.neo4j.gds.core.huge.UnionGraph;
-import org.neo4j.gds.core.loading.construction.GraphFactory;
 import org.neo4j.gds.core.utils.TimeUtil;
 import org.neo4j.gds.utils.ExceptionUtil;
 import org.neo4j.gds.utils.StringJoining;
@@ -115,18 +113,13 @@ public static CSRGraphStore of(
         Map<RelationshipType, RelationshipPropertyStore> relationshipPropertyStores,
         int concurrency
     ) {
-        // A graph store must contain at least one topology, even if it is empty.
-        var topologies = relationships.isEmpty()
-            ? Map.of(RelationshipType.ALL_RELATIONSHIPS, GraphFactory.emptyRelationships(nodes).topology())
-            : relationships;
-
         return new CSRGraphStore(
             databaseId,
             capabilities,
             schema,
             nodes,
             nodePropertyStore == null ? NodePropertyStore.empty() : nodePropertyStore,
-            topologies,
+            relationships,
             relationshipPropertyStores,
             concurrency
         );
@@ -478,27 +471,8 @@ public DeletionResult deleteRelationships(RelationshipType relationshipType) {
     }
 
     @Override
-    public Graph getGraph(Collection<NodeLabel> nodeLabels) {
-        var filteredNodes = getFilteredIdMap(nodeLabels);
-        var filteredNodeProperties = filterNodeProperties(nodeLabels);
-
-        var graphSchema = GraphSchema.of(
-            schema().nodeSchema(),
-            RelationshipSchema.empty(),
-            schema.graphProperties()
-        );
-
-        var initialGraph = HugeGraph.create(
-            nodes,
-            graphSchema,
-            filteredNodeProperties,
-            Relationships.Topology.EMPTY,
-            Optional.empty()
-        );
-
-        return filteredNodes.isPresent()
-            ? new NodeFilteredGraph(initialGraph, filteredNodes.get())
-            : initialGraph;
+    public CSRGraph getGraph(Collection<NodeLabel> nodeLabels) {
+        return getGraph(nodeLabels, List.of(), Optional.empty());
     }
 
     @Override
@@ -508,11 +482,18 @@ public CSRGraph getGraph(
         Optional<String> maybeRelationshipProperty
     ) {
         validateInput(relationshipTypes, maybeRelationshipProperty);
-        return createGraph(nodeLabels, relationshipTypes, maybeRelationshipProperty);
+        if (relationshipTypes.isEmpty()) {
+            return createNodeOnlyGraph(nodeLabels);
+        } else {
+            return createGraph(nodeLabels, relationshipTypes, maybeRelationshipProperty);
+        }
     }
 
     @Override
     public CSRGraph getUnion() {
+        if (relationships.isEmpty()) {
+            return getGraph(nodeLabels());
+        }
         var graphs = relationships
             .keySet()
             .stream()
@@ -681,6 +662,29 @@ private CSRGraph createGraph(
         return UnionGraph.of(filteredGraphs);
     }
 
+    private CSRGraph createNodeOnlyGraph(Collection<NodeLabel> nodeLabels) {
+        var filteredNodes = getFilteredIdMap(nodeLabels);
+        var filteredNodeProperties = filterNodeProperties(nodeLabels);
+
+        var graphSchema = GraphSchema.of(
+            schema().nodeSchema(),
+            RelationshipSchema.empty(),
+            schema.graphProperties()
+        );
+
+        var initialGraph = HugeGraph.create(
+            nodes,
+            graphSchema,
+            filteredNodeProperties,
+            Relationships.Topology.EMPTY,
+            Optional.empty()
+        );
+
+        return filteredNodes.isPresent()
+            ? new NodeFilteredGraph(initialGraph, filteredNodes.get())
+            : initialGraph;
+    }
+
     @NotNull
     private Optional<IdMap> getFilteredIdMap(Collection<NodeLabel> filteredLabels) {
         boolean loadAllNodes = filteredLabels.containsAll(nodeLabels());
@@ -744,13 +748,6 @@ private void validateInput(
         Collection<RelationshipType> relationshipTypes,
         Optional<String> maybeRelationshipProperty
     ) {
-        if (relationshipTypes.isEmpty()) {
-            throw new IllegalArgumentException(formatWithLocale(
-                "The parameter '%s' should not be empty. Use '*' to load all relationship types.",
-                ProcedureConstants.RELATIONSHIP_TYPES
-            ));
-        }
-
         relationshipTypes.forEach(relationshipType -> {
             if (!relationships.containsKey(relationshipType)) {
                 throw new IllegalArgumentException(formatWithLocale(

core/src/test/java/org/neo4j/gds/core/loading/CSRGraphStoreTest.java

@@ -20,10 +20,17 @@
 package org.neo4j.gds.core.loading;
 
 import org.junit.jupiter.api.Test;
+import org.neo4j.gds.NodeLabel;
 import org.neo4j.gds.RelationshipType;
 import org.neo4j.gds.gdl.GdlFactory;
 
+import java.util.List;
+import java.util.Optional;
+
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+import static org.neo4j.gds.TestSupport.assertGraphEquals;
+import static org.neo4j.gds.TestSupport.fromGdl;
 
 class CSRGraphStoreTest {
 
@@ -43,4 +50,36 @@ void deleteAdditionalRelationshipTypes() {
         assertThat(del2.deletedProperties()).isEmpty();
     }
 
+    @Test
+    void validateRelationshipTypesWhenNoneExist() {
+        GdlFactory factory = GdlFactory.of("(a), (b)");
+        var graphStore = factory.build();
+
+        assertThatThrownBy(() -> graphStore.getGraph(
+            List.of(NodeLabel.ALL_NODES),
+            List.of(RelationshipType.of("X")),
+            Optional.empty()
+        )).hasMessageContaining("No relationships have been loaded for relationship type").hasMessageContaining("X");
+    }
+
+    @Test
+    void gettingGraphsWithRelationshipTypes() {
+        GdlFactory factory = GdlFactory.of("()-[:T]->()-[:R]->()-[:R]->()");
+        var graphStore = factory.build();
+
+        var t_graph = graphStore.getGraph(graphStore.nodeLabels(), List.of(RelationshipType.of("T")), Optional.empty());
+        var r_graph = graphStore.getGraph(graphStore.nodeLabels(), List.of(RelationshipType.of("R")), Optional.empty());
+        var t_r_graph = graphStore.getGraph(
+            graphStore.nodeLabels(),
+            List.of(RelationshipType.of("R"), RelationshipType.of("T")),
+            Optional.empty()
+        );
+        var none_graph = graphStore.getGraph(graphStore.nodeLabels(), List.of(), Optional.empty());
+
+        assertGraphEquals(fromGdl("()-[:T]->(), (), ()"), t_graph);
+        assertGraphEquals(fromGdl("(), ()-[:R]->()-[:R]->()"), r_graph);
+        assertGraphEquals(fromGdl("()-[:T]->()-[:R]->()-[:R]->()"), t_r_graph);
+        assertGraphEquals(fromGdl("(), (), (), ()"), none_graph);
+    }
+
 }

proc/community/src/test/java/org/neo4j/gds/wcc/WccStatsProcTest.java

@@ -50,6 +50,15 @@ public WccStatsConfig createConfig(CypherMapWrapper mapWrapper) {
         return WccStatsConfig.of(mapWrapper);
     }
 
+    @Test
+    void testRelationshipTypesEmpty() {
+        runQuery("CALL gds.graph.project('g', '*', '*')");
+
+        assertCypherResult("CALL gds.wcc.stats('g', {relationshipTypes: []}) YIELD componentCount", List.of(Map.of(
+            "componentCount", 10L
+        )));
+    }
+
     @Test
     void yields() {
         loadGraph(DEFAULT_GRAPH_NAME);

test-utils/src/test/java/org/neo4j/gds/extension/TestGraphTest.java

@@ -69,7 +69,7 @@ void usesIdFunctionForOriginalId() {
         CSRGraph bGraph = GdlFactory
             .of("(:A), (b:B), (:B)")
             .build()
-            .getGraph(List.of(NodeLabel.of("B")), List.of(RelationshipType.ALL_RELATIONSHIPS), Optional.empty());
+            .getGraph(List.of(NodeLabel.of("B")), List.of(), Optional.empty());
         TestGraph g = new TestGraph(bGraph, (a) -> a.equals("b") ? 1 : 2, "foo");
         assertEquals(1, g.toOriginalNodeId("b"));
         assertEquals(2, g.toOriginalNodeId("notB"));
@@ -80,7 +80,7 @@ void usesInnerGraphForMappedId() {
         CSRGraph bGraph = GdlFactory
             .of("(:A), (b:B), (:B)")
             .build()
-            .getGraph(List.of(NodeLabel.of("B")), List.of(RelationshipType.ALL_RELATIONSHIPS), Optional.empty());
+            .getGraph(List.of(NodeLabel.of("B")), List.of(), Optional.empty());
         TestGraph g = new TestGraph(bGraph, (a) -> a.equals("b") ? 1 : 2, "foo");
         assertEquals(0, g.toMappedNodeId("b"));
         assertEquals(1, g.toMappedNodeId("notB"));