Fix union of models or references

packages/mst-query/package.json

@@ -1,6 +1,6 @@
 {
     "name": "mst-query",
-    "version": "4.2.0",
+    "version": "4.2.1",
     "description": "Query library for mobx-state-tree",
     "source": "src/index.ts",
     "type": "module",

packages/mst-query/src/utils.ts

@@ -32,7 +32,14 @@ export function getSubType(t: any, data?: any): any {
             return getSubType(t._subtype);
         }
         const subTypes = t._types.map((t: any) => getSubType(t, data));
-        const modelWithProperties = subTypes.find((x: any) => isModelType(x) || isReferenceType(x));
+        // Every subtype is a model type or reference type - return the union type and let mst
+        // handle reconciling the types.
+        if (subTypes.every((x: any) => isModelType(x) || isReferenceType(x))) {
+            return t;
+        }
+        // If we have a union of models and primitives (null/undefined), we need to find the first model or reference type
+        // to enumerate the properties of the object.
+        const modelWithProperties = subTypes.find((x: any) => isModelType(x) || isReferenceType(x));        
         if (modelWithProperties) {
             return getSubType(modelWithProperties, data);
         }

packages/mst-query/tests/mstQuery.test.tsx

@@ -1307,3 +1307,35 @@ test('initial data should only be set on mount', async () => {
 
     configureMobx({ enforceActions: 'observed' });
 });
+
+test('union of array models', () => {
+    const data = {
+        rules: [
+            {
+                kind: 'FIXED',
+                fixedValue: 'Fixed value',
+            },
+            {
+                kind: 'FORMAT',
+                formatValue: 'Formatted value',
+            },            
+        ],
+    };
+    const Model = types.model('UnionArrayTestModel', {
+        rules: types.array(
+            types.union(
+                types.late(() => types.model('FixedModel', {
+                    kind: types.literal('FIXED'),
+                    fixedValue: types.string,
+                })),
+                types.model('FormatModel', {
+                    kind: types.literal('FORMAT'),
+                    formatValue: types.string,
+                }),
+            ),
+        ),
+    });
+    const result = merge(data, Model, {});
+    expect(result.rules[0].fixedValue).toBe('Fixed value');
+    expect(result.rules[1].formatValue).toBe('Formatted value');
+});