如何使用nodejs合并mongodb中的多个集合

Question

Trying to merge multiple collection using nodejs and mongoose but not working.Anyone can find solution for this.

Getting this error:

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

product.model.js:

    module.exports = mongoose.model('Test1', userSchemaTest1, 'test1');
    module.exports = mongoose.model('Test2', userSchemaTest2, 'test2');
    module.exports = mongoose.model('Test3', userSchemaTest3, 'test3');
    module.exports = mongoose.model('Test4', userSchemaTest4, 'test4');
    module.exports = mongoose.model('Test5', userSchemaTest5, 'test5');

product.controller.js:

module.exports.getAllProducts = (req, res, next) => {

    let collection = req.query.collection;
    var newVal = collection.split(',');
    var allP = [];
    var getAllp;
    allP.push(...newVal);
    allP.forEach((element) => {
        let tabledatas = mongoose.model(element);
        tabledatas.find({}, function(err, docs) {
            if (err) {
                console.log('ss' + err);
                return
            }
            getAllp = [...getAllp, res.json(docs)];
        })
    })
    return getAllp; 

}

api call

http://localhost:3000/api/getAllProducts?collection=Test1,Test2,Test3,Test4,Test5 

Answer 1

There are several issues with your code, I'll focus only on how to combine results of multiple queries, which is actually how to deal with async function results in general. But you should take notes of other issues mentioned here as well

• Calling res.json() in a loop, you'll certainly get "Headers already sent" errors
• Putting the result of res.json() in to an array; res.json() returns ServerResponse , which has nothing to do with what we want.
• Accepting user inputs (collection names) and pass them directly to the database operations is dangerous.
• Handling async functions:

// models would be a list of model names
const models = collection.split(',')

// map array of model names to array of Promises (result of async operations)
const resultPromises = models.map(modelName => {
  return mongoose.model(modelName).find({}).exec() // use Promise result instead of passing callback
})

// wait on all Promises to be fulfilled and resolved to actual results.
const combinedResults = await Promise.all(resultPromises)
// now you will get a result of this form [[item1, item2, ...], [itemX, itemY, ...], ...]
// but what you want is [item1, item2, ..., itemX, itemY, ...], we can use array.flat()

cont results = combinedResults.flat()

Note that you can only use await in an async function, so you'll have to modify your function like this

module.exports.getAllProducts = async (req, res, next) => { // add async keyword
  // ...
}

more on array.flat()