(graphache) - Fix query traversal for previous null fields

.changeset/slimy-chairs-sniff.md

@@ -0,0 +1,5 @@
+---
+'@urql/exchange-graphcache': patch
+---
+
+Fix traversal issue, where when a prior selection set has set a nested result field to `null`, a subsequent traversal of this field attempts to access `prevData` on `null`.

exchanges/graphcache/src/operations/query.test.ts

@@ -187,4 +187,65 @@ describe('Query', () => {
       todos: [{ __typename: 'Todo', id: '0', text: 'Solve bug' }],
     });
   });
+
+  it('should allow subsequent read when first result was null', () => {
+    const QUERY_WRITE = gql`
+      query writeTodos {
+        todos {
+          ...ValidRead
+        }
+      }
+
+      fragment ValidRead on Todo {
+        id
+      }
+    `;
+
+    const QUERY_READ = gql`
+      query getTodos {
+        todos {
+          ...MissingRead
+        }
+        todos {
+          id
+        }
+      }
+
+      fragment ValidRead on Todo {
+        id
+      }
+
+      fragment MissingRead on Todo {
+        id
+        text
+      }
+    `;
+
+    const store = new Store({
+      schema: alteredRoot,
+    });
+
+    let { data } = query(store, { query: QUERY_READ });
+    expect(data).toEqual(null);
+
+    write(
+      store,
+      { query: QUERY_WRITE },
+      {
+        todos: [
+          {
+            __typename: 'Todo',
+            id: '0',
+          },
+        ],
+        __typename: 'Query',
+      }
+    );
+
+    ({ data } = query(store, { query: QUERY_READ }));
+    expect(data).toEqual({
+      __typename: 'query_root',
+      todos: [null],
+    });
+  });
 });

exchanges/graphcache/src/operations/query.ts

@@ -391,7 +391,7 @@ const resolveResolverResult = (
   fieldName: string,
   key: string,
   select: SelectionSet,
-  prevData: void | Data | Data[],
+  prevData: void | null | Data | Data[],
   result: void | DataField
 ): DataField | void => {
   if (Array.isArray(result)) {
@@ -410,7 +410,7 @@ const resolveResolverResult = (
         joinKeys(key, `${i}`),
         select,
         // Get the inner previous data from prevData
-        prevData !== undefined ? prevData[i] : undefined,
+        prevData ? prevData[i] : undefined,
         result[i]
       );
 
@@ -424,8 +424,12 @@ const resolveResolverResult = (
     return data;
   } else if (result === null || result === undefined) {
     return result;
+  } else if (prevData === null) {
+    // If we've previously set this piece of data to be null,
+    // we skip it and return null immediately
+    return null;
   } else if (isDataOrKey(result)) {
-    const data = (prevData === undefined ? {} : prevData) as Data;
+    const data = (prevData || {}) as Data;
     return typeof result === 'string'
       ? readSelection(ctx, result, select, data)
       : readSelection(ctx, key, select, data, result);
@@ -448,7 +452,7 @@ const resolveLink = (
   typename: string,
   fieldName: string,
   select: SelectionSet,
-  prevData: void | Data | Data[]
+  prevData: void | null | Data | Data[]
 ): DataField | undefined => {
   if (Array.isArray(link)) {
     const { store } = ctx;
@@ -462,7 +466,7 @@ const resolveLink = (
         typename,
         fieldName,
         select,
-        prevData !== undefined ? prevData[i] : undefined
+        prevData ? prevData[i] : undefined
       );
       if (childLink === undefined && !_isListNullable) {
         return undefined;
@@ -472,15 +476,12 @@ const resolveLink = (
     }
 
     return newLink;
-  } else if (link === null) {
+  } else if (link === null || prevData === null) {
+    // If the link is set to null or we previously set this piece of data to be null,
+    // we skip it and return null immediately
     return null;
   } else {
-    return readSelection(
-      ctx,
-      link,
-      select,
-      prevData === undefined ? ({} as any) : prevData
-    );
+    return readSelection(ctx, link, select, (prevData || {}) as Data);
   }
 };