How can I use one _id between two collections when joining two collection
If you are using mongoose, it has a populate method built in. Consider changing your Word schema to:
const WordSchema = new Schema({ word:{ type:String, }, translate: { type:String, }, kind:{ type:String, }, exampleSentence: { type:String, }, isCorrect: { type:Boolean }, event: { type:mongoose.Schema.ObjectId, ref: 'Test', required:true }});Then, instead of using lookup, do (where Word is a reference to the WordSchema):
router.get("/", async (req,res) => { const data = await Word.find().populate('Test').exec(); res.json(data);}) Populate will automatically populate your references for you.