✨ Support filtering on linked nodes

The filtering is done by extracting all the related nodes using `extractFieldExample`. The example values are then cached to save some time. (As of this patch, caching is not tested automatically)

Further linking are disabled to avoid cyclic dependencies (input fields only).

TODO: Filtering on nodes linked by mappings and File

Note: If linking is done via array of IDs, only the structure of the first item is extracted. I'll be happy to add union support if someone can show me how to `$in` filter on an array of object.

Related gatsbyjs#3613 gatsbyjs#3190

packages/gatsby/src/schema/__tests__/infer-graphql-input-type-test.js

@@ -483,3 +483,63 @@ describe(`GraphQL Input args`, () => {
     expect(result).toMatchSnapshot()
   })
 })
+
+describe(`filtering on linked nodes`, () => {
+  let types
+  beforeEach(() => {
+    const { store } = require(`../../redux`)
+    types = [
+      {
+        name: `Child`,
+        nodeObjectType: new GraphQLObjectType({
+          name: `Child`,
+          fields: inferObjectStructureFromNodes({
+            nodes: [{ id: `child_1`, hair: `brown`, height: 101 }],
+            types: [{ name: `Child` }],
+          }),
+        }),
+      },
+      {
+        name: `Pet`,
+        nodeObjectType: new GraphQLObjectType({
+          name: `Pet`,
+          fields: inferObjectStructureFromNodes({
+            nodes: [{ id: `pet_1`, species: `dog` }],
+            types: [{ name: `Pet` }],
+          }),
+        }),
+      },
+    ]
+
+    store.dispatch({
+      type: `CREATE_NODE`,
+      payload: { id: `child_1`, internal: { type: `Child` }, hair: `brown` },
+    })
+    store.dispatch({
+      type: `CREATE_NODE`,
+      payload: { id: `child_2`, internal: { type: `Child` }, hair: `blonde`, height: 101 },
+    })
+    store.dispatch({
+      type: `CREATE_NODE`,
+      payload: { id: `pet_1`, internal: { type: `Pet` }, species: `dog` },
+    })
+  })
+
+  it(`filters on linked nodes via id`, async () => {
+    let result = await queryResult(
+      [{ linked___NODE: `child_2`, foo: `bar` }, { linked___NODE: `child_2`, foo: `baz` }],
+      `
+        {
+          allNode(filter: { linked: { hair: { eq: "blonde" } } }) {
+            edges { node { linked { hair, height }, foo } }
+          }
+        }
+      `,
+      { types }
+    )
+    expect(result.data.allNode.edges[0].node.linked.hair).toEqual(`blonde`)
+    expect(result.data.allNode.edges[0].node.linked.height).toEqual(101)
+    expect(result.data.allNode.edges[0].node.foo).toEqual(`bar`)
+    expect(result.data.allNode.edges[1].node.foo).toEqual(`baz`)
+  })
+})

packages/gatsby/src/schema/infer-graphql-input-fields.js

@@ -19,6 +19,9 @@ const {
   isEmptyObjectOrArray,
 } = require(`./data-tree-utils`)
 
+const { findLinkedNode } = require(`./infer-graphql-type`)
+const { getNodes } = require(`../redux`)
+
 import type {
   GraphQLInputFieldConfig,
   GraphQLInputFieldConfigMap,
@@ -185,6 +188,8 @@ type InferInputOptions = {
   exampleValue?: Object,
 }
 
+const linkedNodeCache = {}
+
 export function inferInputObjectStructureFromNodes({
   nodes,
   typeName = ``,
@@ -196,13 +201,34 @@ export function inferInputObjectStructureFromNodes({
 
   prefix = isRoot ? typeName : prefix
 
-  _.each(exampleValue, (value, key) => {
+  _.each(exampleValue, (v, k) => {
+    let value = v
+    let key = k
     // Remove fields for traversing through nodes as we want to control
     // setting traversing up not try to automatically infer them.
     if (isRoot && EXCLUDE_KEYS[key]) return
 
-    // Input arguments on linked fields aren't currently supported
-    if (_.includes(key, `___NODE`)) return
+    if (_.includes(key, `___NODE`)) {
+      // TODO: Union the objects in array
+      const nodeToFind = _.isArray(value) ? value[0] : value
+      const linkedNode = findLinkedNode(nodeToFind)
+
+      // Get from cache if found, else store into it
+      if (linkedNodeCache[linkedNode.internal.type]) {
+        value = linkedNodeCache[linkedNode.internal.type]
+      } else {
+        const relatedNodes = getNodes().filter(node => node.internal.type === linkedNode.internal.type)
+        value = extractFieldExamples(relatedNodes)
+        value = _.omitBy(value, (_v, _k) => _.includes(_k, `___NODE`))
+        linkedNodeCache[linkedNode.internal.type] = value
+      }
+
+      if (_.isArray(value)) {
+        value = [value]
+      }
+
+      ;[key] = key.split(`___`)
+    }
 
     let field = inferGraphQLInputFields({
       nodes,

packages/gatsby/src/schema/infer-graphql-type.js

@@ -252,7 +252,7 @@ function inferFromMapping(
   }
 }
 
-function findLinkedNode(value, linkedField, path) {
+export function findLinkedNode(value, linkedField, path) {
   let linkedNode
   // If the field doesn't link to the id, use that for searching.
   if (linkedField) {