How to compare 2 collections and fetch matched field data in mongodb? - arrays

Question

I have a two collection as given below.
Collection 1: List
{
"_id" : ObjectId("5b618589d6edcc135c1aee0a"),
"name" : "ABC",
"special" : "Golmal",
"dim" : "2122",
}
Collection 2: Data
Document 1:
{
"_id" : ObjectId("5b6023d9589ff6bfb5dd75d7"),
"date" : "2018/08/13",
"special" : "Golmal",
}
Document 2:
{
"_id" : ObjectId("5b602447589ff6bfb5dd7606"),
"date" : "2018/08/13",
"special" : "Godman",
}
Here I want compare both the collection and if "special":"Golmal" of Collection 1 is present in Collection 2 documents I want to display/fetch all the related date from Collection 1 document.
I have tried aggregate & $lookup as below
getAvailableList(): Promise<any> {
return this._mongoUtility.Db
.then(db => {
let list= db.collection('list');
let data= db.collection('data');
list.aggregate([{$lookup:
{
from: data,
localField: "special",
foreignField: "special",
as: "matches"
}
},
{
$match: { "matches": { $ne: [{}] } }
}
]);
return Promise.resolve();
})
.catch(err => {
return Promise.reject({message: 'Not found', err: err});
});
}
But I'm getting null.
I have also tied by using .find & forEach as below
let abc = [];
list.find({}).toArray()
.then(arr => {
arr.forEach(obj1 => {
data.find({special: obj1.special}).toArray()
.then(secondArr => {
console.log(secondArr);
if (secondArr.length > 0) {
abc.push(obj1);
}
});
});
});
return Promise.resolve();
But no result. I need value in JSON format. As MongoDB dont have joins. Is there any ways to crack it?

Answer 1

Well, I changed my way of doing that query by not using forEach and its working for me.
I come across $in in find in mongoDB website. Here's the answer
return new Promise((res, rej) => {
this._mongoUtility.Db
.then(db => {
let list= db.collection('List');
let data= db.collection('Data');
data.find({}).toArray()
.then(arr => {
let maps = arr.map(data => data.special);
list.find({special: {$in: maps}}).toArray()
.then(secondArr => {
res(secondArr);
});
});
})
.catch(err => {
return rej({message: 'Not found', err: err});
});
});

Answer 2

As of MongoDB 3.2, joins become possible.
https://www.mongodb.com/blog/post/joins-and-other-aggregation-enhancements-coming-in-mongodb-3-2-part-1-of-3-introduction