UPDATING ARRAYS IN MONGODB DOCUMENTS

MongoDB is such a great database program that once you use it, it is hard going back. One pain in the backside though that I’ve always experienced is updating arrays inside mongoDB documents. For some reason or the other this is something that always bothered me.

So, for all the people like me who are sailing in the same boat maybe these examples and code snippets will help you (and me in the future) to update array elements properly in mongo.

Updating matched array elements

Assume that there is mongoDB collection called cards where each document has a field called attacks. This attacks field is an array of objects that contains the various attacks that the card can do and their attackPoints. Something along the lines of :-

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{ attacks: [{     attack: "Fireball",     attackPoints: 45   },   {     attack: "Fire Blast",     attackPoints: 60   }   ] }

Suppose we want to change the Fireball attack to have attackPoints as 40 for all the cards that have this Fireball attack. The query that we would write for this is :-

1	db.cards.updateMany({attacks: {$elemMatch : { attack: "Fireball", attackPoints: 45}}}, {$set: {"hobbies.$": {attack: "Fireball", attackPoints: 40}}});

Some points to keep note of here is that if we had used just “hobbies” inside the $set function we would have replaced the entire attacks array with this object. It is the $ after the “hobbies.” that makes mongo realise that only the matching element need to be updated.

Updating all array elements

Another thing you might want to do is updating a single field of all the elements in the array. Let us assume we want to increase the attackPoints field by 5 for every attack of every card.

1	db.cards.updateMany({}, {$inc : { "attacks.$[].attackPoints": 5 }});

Another new thing here is the $[] , what this notation tells mongo to do is to update all the elements in the array.

Adding and removing elements

Let us see how to push a single element and multiple elements into a document array :-

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//Single Element db.cards.updateMany({},{$push: {attacks: {attack: "Ice Beam", attackPoints: 20}}});   //Multiple Elements db.cards.updateMany({},{$push: {attacks: {$each : [{attack: "Earthquake", attackPoints: 30},{attack: "Zap", attackPoints: "44"}]}}});

The $push identifier pushes the elements onto the arrays while the $each identifier loops over the elements and does some operation on each of them (here, pushing them onto the array).

Removing elements from an array

The final thing with array updation is removing elements from the arrays. We can do that in two different ways – one using the $pull identifier and the other using the $pop identifier.

The different between these two is that the $pull method allows you to give filters to search for and remove elements while the $pop identifier will remove only the first or the last element of the array.

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// Using pop db.cards.updateMany({},{$pop: {attacks: 1}});   // Using pull db.cards.updateMany({},{$pull : {attacks: { attack: "Zap" }}});

Note that the $pop can take in either 1 or -1 as the value after you mention the array. 1 is for the last element and -1 is to remove the first element.