almost there !

src/gql-graph/buildDocumentSelection.ts

@@ -18,7 +18,7 @@ export const buildDocumentSelection = (qObj: GraphQueryObj): DocumentSelection |
     return null
   }
   const documentValueFieldsSelections = qObj.select.reduce((_documentSelection, selection) => {
-    const valueField: ValueField = { fieldName: selection.field }
+    const valueField: ValueField = selection
     return {
       ..._documentSelection,
       [selection.alias]: valueField,
@@ -27,7 +27,7 @@ export const buildDocumentSelection = (qObj: GraphQueryObj): DocumentSelection |
   const documentUnionLookupFieldSelections = qObj.traverse.reduce(
     (_documentSelection, qObjSelection) => {
       const _existingAlias = _documentSelection[qObjSelection.alias]
-      if (_existingAlias && 'fieldName' in _existingAlias) {
+      if (_existingAlias && !isUnionLookupField(_existingAlias)) {
         throw aliasConflictError(qObjSelection, qObj)
       }
       const select = buildDocumentSelection(qObjSelection)
@@ -37,7 +37,13 @@ export const buildDocumentSelection = (qObj: GraphQueryObj): DocumentSelection |
 
       const unionLookupField: UnionLookupField =
         _existingAlias ||
-        (_documentSelection[qObjSelection.alias] = { page: qObjSelection.page, lookups: [] })
+        (_documentSelection[qObjSelection.alias] = {
+          page: qObjSelection.page,
+          fieldName: qObjSelection.fieldName,
+          alias: qObjSelection.alias,
+          lookups: [],
+          traverseRelation: qObjSelection.traverseRelation,
+        })
 
       unionLookupField.lookups.push({
         __typename: qObjSelection.__typename,
@@ -63,5 +69,4 @@ const aliasConflictError = (selection: GraphQueryObj, par: GraphQueryObj) => {
   return new Error(`A ValueField can't have same alias[${selection.alias}] as a UnionLookupField`)
 }
 
-export const isValueField = (_: Field): _ is ValueField => !!(_ && 'fieldName' in _)
 export const isUnionLookupField = (_: Field): _ is UnionLookupField => !!(_ && 'lookups' in _)

src/gql-graph/defaultGraphFieldResolver.ts

@@ -2,10 +2,11 @@ import { Context } from '../gql'
 import { ResolverFn } from '../gql/types'
 import { ShallowTypeMocks } from '../gql/shallowTypes'
 import { typeInfo, getParent } from './graphQuery'
-import { GraphQueryObj } from './types'
+import { GraphQueryObj, TraverseRelation } from './types'
 
 export function defaultGraphFieldResolver(): ResolverFn<any, any, Context, any> {
   return (parent, args, context, info) => {
+    const { fieldName, parentType, returnType, schema } = info
     if (!context.$graph) {
       context.$graph = {
         qObj: {
@@ -13,11 +14,12 @@ export function defaultGraphFieldResolver(): ResolverFn<any, any, Context, any>
           alias: 'Query',
           directives: {},
           traverse: [],
+          traverseRelation: null,
+          fieldName: fieldName,
           select: [],
         },
       }
     }
-    const { fieldName, parentType, returnType, schema } = info
     console.log(`defaultFieldResolver `, {
       parent,
       parentTypeName: parentType.name,
@@ -37,7 +39,7 @@ export function defaultGraphFieldResolver(): ResolverFn<any, any, Context, any>
     if (!parentQ) {
       throw new Error('defaultFieldResolver no parentQ')
     }
-    if (isTop || ['_rel', '_subj', '_obj'].includes(fieldName)) {
+    if (isTop || isTraverseRelation(fieldName)) {
       parentQ.traverse.push(
         ...__typenames.map<GraphQueryObj>((__typename) => ({
           __typename,
@@ -46,7 +48,9 @@ export function defaultGraphFieldResolver(): ResolverFn<any, any, Context, any>
           page: args?.page,
           select: [],
           traverse: [],
+          traverseRelation: isTraverseRelation(fieldName) ? fieldName : null,
           directives: {},
+          fieldName: fieldName,
         }))
       )
 
@@ -62,9 +66,12 @@ export function defaultGraphFieldResolver(): ResolverFn<any, any, Context, any>
       return fieldReturn
     } else {
       if ('string' == typeof info.path.key) {
-        parentQ.select.push({ field: fieldName, alias: info.path.key })
+        parentQ.select.push({ fieldName: fieldName, alias: info.path.key })
       }
       return parent[fieldName]
     }
   }
 }
+
+const isTraverseRelation = (_: string): _ is TraverseRelation =>
+  ['_subj', '_obj', '_rel'].includes(_)

src/gql-graph/mongo/buildMongoPipeline.ts

 import { DocumentSelection } from '../types'
-import { isValueField } from '../queryBuilder'
-export const buildMongoPipeline = (docS: DocumentSelection) => {
+import { isUnionLookupField } from '../buildDocumentSelection'
+export const buildMongoPipeline = (docS: DocumentSelection, notTop?: boolean) => {
   const lookups = [] as any[]
   const project = {} as Record<string, string>
 
   Object.entries(docS).forEach(([alias, field]) => {
-    if (isValueField(field)) {
-      project[field.fieldName] = alias
-    } else {
-      const $match = { $and: [] as any[] }
-      const $unionWith = {
-        coll: 'Graph',
-        pipeline: [{ $match }] as any[],
-      }
+    if (isUnionLookupField(field)) {
+      const $mainMatchUnionMatch = { $or: [] as any[] }
+      const $mainMatch = { $and: [$mainMatchUnionMatch] }
+      const pipeline = [{ $match: $mainMatch }] as any[]
       const $lookup = {
-        as: alias,
+        as: field.alias,
+        let: {} as Record<string, string>,
         from: 'Graph',
-        pipeline: [{ $unionWith }],
+        pipeline,
+      }
+
+      if (field.traverseRelation) {
+        if (field.traverseRelation === '_rel') {
+          $lookup.let.nodeId = '$_id'
+
+          notTop &&
+            $mainMatch.$and.push({
+              // $expr: { $eq: [`$${field.traverseRelation}`, '$$nodeId'] },
+              $expr: { $eq: [`$_subj`, '$$nodeId'] },
+            } as any)
+        } else {
+          $lookup.let.edgeSide = `$${field.traverseRelation}` //=== '_obj' ? '$_subj' : '$_obj'
+          notTop && $mainMatch.$and.push({ $expr: { $eq: [`$_id`, '$$edgeSide'] } } as any)
+        }
       }
+
       field.lookups.forEach((fieldLookup) => {
-        $match.$and.push({ __typename: fieldLookup.__typename })
+        const $match = { $and: [{ __typename: fieldLookup.__typename }] as any[] }
+        $mainMatchUnionMatch.$or.push($match)
         fieldLookup.match && $match.$and.push(fieldLookup.match)
-        const pipeline = buildMongoPipeline(fieldLookup.select)
-        $unionWith.pipeline.push(...pipeline)
+        pipeline.push(...buildMongoPipeline(fieldLookup.select, true))
       })
+      project[field.alias] = `$${field.alias}`
       lookups.push({
         $lookup,
       })
+    } else {
+      project[alias] = `$${field.fieldName}`
     }
   })
-  const stages = [...lookups]
-  if (Object.keys(project).length) {
-    stages.push({ $project: project })
+  if (notTop) {
+    const stages = [...lookups]
+    if (Object.keys(project).length) {
+      stages.push({ $project: project })
+    }
+    return stages
+  } else {
+    const stages = lookups[0].$lookup.pipeline
+    return stages
   }
-  return stages
 }

src/gql-graph/types.d.ts

@@ -7,14 +7,17 @@ export type GqlType = { id: string; __typename: string }
 export type GqlNode = GqlType & { _rel: any[] }
 export type GqlRelation = GqlType & { _obj: any; _subj: any }
 
+export type TraverseRelation = '_obj' | '_subj' | '_rel'
+
 // GraphQuery
 
 export type GraphQuery = {
   qObj: GraphQueryObj
 }
 
-export type GraphQueryObj = {
+export type GraphQueryObj = Selection & {
   __typename: string
+  traverseRelation: TraverseRelation | null
   match?: any
   page?: {
     limit?: number
@@ -23,11 +26,10 @@ export type GraphQueryObj = {
   directives: Record<string, any>
   select: Selection[]
   traverse: GraphQueryObj[]
-  alias: string
 }
 
 export type Selection = {
-  field: string
+  fieldName: string
   alias: string
 }
 export type Match = any
@@ -41,15 +43,14 @@ export type DocumentSelection = {
 
 export type Field = UnionLookupField | ValueField
 
-export type ValueField = {
-  fieldName: string
-}
+export type ValueField = Selection
 
-export type UnionLookupField = {
+export type UnionLookupField = ValueField & {
   page?: {
     limit?: number
     after?: string | ObjectID
   }
+  traverseRelation: TraverseRelation | null
   lookups: FieldLookup[]
 }
 

src/http/index.ts

@@ -3,8 +3,8 @@ import { ApolloServer } from 'apollo-server-express'
 import express from 'express'
 import { graphql, GraphQLSchema } from 'graphql'
 import { Context } from '../gql'
-import { buildMongoPipeline } from '../gql-graph/mongo/buildMongoQuery'
-import { buildQueryDocumentSelection } from '../gql-graph/queryBuilder'
+import { buildMongoPipeline } from '../gql-graph/mongo/buildMongoPipeline'
+import { buildQueryDocumentSelection } from '../gql-graph/buildDocumentSelection'
 import { collection } from '../mongo/collection'
 
 type Cfg = {
@@ -28,7 +28,7 @@ const executor: GraphQLServerOptions['executor'] = async (requestContext) => {
   console.dir({ 'executor documentSelection': documentSelection }, { depth: 15 })
   if (documentSelection) {
     const pipeline = buildMongoPipeline(documentSelection)
-    console.dir({ 'executor pipeline': pipeline }, { depth: 15 })
+    console.log('executor pipeline', JSON.stringify(pipeline, null, 4))
     const c = await collection<any>()
     return { data: await c.aggregate(pipeline).toArray() }
   }