CODEWITHSUNDEEP
mongodbmongodb-queryaggregation-framework
joshuwa
joshuwa

Reputation: 13

Match every object in an array in MongoDB

I have an author collection that looks like this: 

{ "_id" : ObjectId("332ddf"), 
  "authors" : "Mark Twain", 
  "publisher" : "NY", 
  "books" : [ "The Adventures of Tom Sawyer", "The Prince and the Pauper" ] }

{ "_id" : ObjectId("4ef342"), 
  "authors" : "F. Scott Fitzgerald", 
  "publisher" : "NY", 
  "books" : [ "The Adventures of Tom Sawyer", 
              "The Great Gatsby", "This Side of Paradise" ] }

I am trying to use the aggregation to create a new collection. For aggregation I tried: 

db.author.aggregate([
   { "$unwind": "$books"},
   { $project: 
     { 
        book: "$books",
        authors: ["$authors"]

     }
     }
])

but I get duplicate key error collection for unwinding the books. How can I get the desired collection using aggregation ?

Upvotes: 1

Views: 80

Answers (1)

Sede
Sede

Reputation: 61225

Actually you don't need the $project stage. All you need to do is "denormalize" the "Books" array using the $unwind operator then $group your documents by "book" and use the $push operator to return an array of "authors".

var cursor = db.getCollection('authors').aggregate([
    { "$unwind": "$books" }, 
    { "$group": { 
        "_id": "$books", 
        "authors": { "$push": "$authors" }
    }}
])

The aggregation query yields something like this:

{ "_id" : "This Side of Paradise", "authors" : [ "F. Scott Fitzgerald" ] }
{ "_id" : "The Great Gatsby", "authors" : [ "F. Scott Fitzgerald" ] }
{ "_id" : "The Prince and the Pauper", "authors" : [ "Mark Twain" ] }
{
        "_id" : "The Adventures of Tom Sawyer",
        "authors" : [
                "Mark Twain",
                "F. Scott Fitzgerald"
        ]
}

BTW the _id compound field in your expected result doesn't make much sense so I removed it but if you really feel you need it then simply replace "_id": "$books" by "_id": { "book": "$books" } in the group stage.

Now let see how we can insert into another collection. one way to do this as mention in the comment below is using the $out operator which must be the last stage in the aggregation pipeline.

{ "$out": "newCollection" }

In case you need to process the result client-side before inserting the document into a new collection you should use "bulk" operation.

MongoDB 3.2 or newer

var requests = [];
var count = 0;

cursor.forEach(function(document) {
    // Do something with the document and push a new operation to the stack
    requests.push({ insertOne : document });
    count++;
    if(count % 1000 === 0) {
        db.newCollection.bulkWrite(requests);
        requests = [];
        count = 0;
    }
});

db.newCollection.bulkWrite(requests);

MongoDB 3.0 backwards

var bulk = db.newCollection.initializeUnorderedBulkOp();
var count = 0;

cursor.forEach(function(document) {
    // Do something with the document and push a new operation to the stack
    bulk.insert(document);
    count++;
    if (count % 1000 === 0) {
        bulk.execute(); 
        bulk = db.newCollection.initializeUnorderedBulkOp();
    }
})

if (count > 0 )  {
    bulk.execute();
}

Upvotes: 2

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