Fix error message in single-root-entity-fields

Previously, this was [object object].

spec/regression/logistics/tests/query-single.graphql

@@ -0,0 +1,52 @@
+query byID {
+    Delivery(id: "@{ids/Delivery/3}") {
+        deliveryNumber
+    }
+}
+
+query byKey {
+    Delivery (deliveryNumber: "1000521") {
+        deliveryNumber
+    }
+}
+
+query notFound {
+    Delivery (deliveryNumber: "does-not-exist") {
+        deliveryNumber
+    }
+}
+
+# throws
+query noArgs {
+    Delivery {
+        deliveryNumber
+    }
+}
+
+# throws
+query tooManyArgs {
+    Delivery(id: "@{ids/Delivery/3}", deliveryNumber: "1000521") {
+        deliveryNumber
+    }
+}
+
+# currently returns null, but should probably be an error
+query nullArg {
+    Delivery(id: null) {
+        deliveryNumber
+    }
+}
+
+# throws
+query twoNullArgs {
+    Delivery(id: null, deliveryNumber: null) {
+        deliveryNumber
+    }
+}
+
+# currently throws, but should probably be ok
+query nullAndNonNullArg {
+    Delivery(id: null, deliveryNumber: "1000521") {
+        deliveryNumber
+    }
+}

spec/regression/logistics/tests/query-single.result.json

@@ -0,0 +1,102 @@
+{
+  "byID": {
+    "data": {
+      "Delivery": {
+        "deliveryNumber": "1000522"
+      }
+    }
+  },
+  "byKey": {
+    "data": {
+      "Delivery": {
+        "deliveryNumber": "1000521"
+      }
+    }
+  },
+  "notFound": {
+    "data": {
+      "Delivery": null
+    }
+  },
+  "noArgs": {
+    "errors": [
+      {
+        "message": "Must specify exactly one argument to field \"Delivery\"",
+        "locations": [
+          {
+            "line": 21,
+            "column": 5
+          }
+        ],
+        "path": [
+          "Delivery"
+        ]
+      }
+    ],
+    "data": {
+      "Delivery": null
+    }
+  },
+  "tooManyArgs": {
+    "errors": [
+      {
+        "message": "Must specify exactly one argument to field \"Delivery\"",
+        "locations": [
+          {
+            "line": 28,
+            "column": 5
+          }
+        ],
+        "path": [
+          "Delivery"
+        ]
+      }
+    ],
+    "data": {
+      "Delivery": null
+    }
+  },
+  "nullArg": {
+    "data": {
+      "Delivery": null
+    }
+  },
+  "twoNullArgs": {
+    "errors": [
+      {
+        "message": "Must specify exactly one argument to field \"Delivery\"",
+        "locations": [
+          {
+            "line": 42,
+            "column": 5
+          }
+        ],
+        "path": [
+          "Delivery"
+        ]
+      }
+    ],
+    "data": {
+      "Delivery": null
+    }
+  },
+  "nullAndNonNullArg": {
+    "errors": [
+      {
+        "message": "Must specify exactly one argument to field \"Delivery\"",
+        "locations": [
+          {
+            "line": 49,
+            "column": 5
+          }
+        ],
+        "path": [
+          "Delivery"
+        ]
+      }
+    ],
+    "data": {
+      "Delivery": null
+    }
+  }
+}

src/schema-generation/utils/entities-by-uinque-field.ts

@@ -13,7 +13,7 @@ export function getEntitiesByUniqueFieldQuery(rootEntityType: RootEntityType, ar
     const filterClauses = objectEntries(args).map(([fieldName, value]) =>
         new BinaryOperationQueryNode(createFieldNode(rootEntityType.getFieldOrThrow(fieldName), entityVarNode), BinaryOperator.EQUAL, new LiteralQueryNode(value)));
     if (filterClauses.length != 1) {
-        throw new Error(`Must specify exactly one argument to ${rootEntityType.toString()}`); // TODO throw this at the correct GraphQL query location
+        throw new Error(`Must specify exactly one argument to field "${rootEntityType.name}"`); // TODO throw this at the correct GraphQL query location
     }
     const filterNode = filterClauses[0];
     const listNode = new EntitiesQueryNode(rootEntityType);