How to remove an element from a doubly-nested array in a MongoDB document.

Question

I have a document structure something along the lines of the following:

{ "_id" : "777", "someKey" : "someValue", "someArray" : [     {         "name" : "name1",         "someNestedArray" : [             {                 "name" : "value"             },             {                 "name" : "delete me"             }         ]     }   ] } 

I want to delete the nested array element with the value "delete me".

I know I can find documents which match this description using nested $elemMatch expressions. What is the query syntax for removing the element in question?

Answer 1

To delete the item in question you're actually going to use an update. More specifically you're going to do an update with the $pull command which will remove the item from the array.

db.temp.update(   { _id : "777" },   {$pull : {"someArray.0.someNestedArray" : {"name":"delete me"}}} ) 

There's a little bit of "magic" happening here. Using .0 indicates that we know that we are modifying the 0th item of someArray. Using {"name":"delete me"} indicates that we know the exact data that we plan to remove.

This process works just fine if you load the data into a client and then perform the update. This process works less well if you want to do "generic" queries that perform these operations.

I think it's easiest to simply recognize that updating arrays of sub-documents generally requires that you have the original in memory at some point.

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In response to the first comment below, you can probably help your situation by changing the data structure a little

"someObjects" : {   "name1":  {         "someNestedArray" : [             {                 "name" : "value"             },             {                 "name" : "delete me"             }         ]     } } 

Now you can do {$pull : { "someObjects.name1.someNestedArray" : ...

Here's the problem with your structure. MongoDB does not have very good support for manipulating "sub-arrays". Your structure has an array of objects and those objects contain arrays of more objects.

If you have the following structure, you are going to have a difficult time using things like $pull:

array [   { subarray : array [] },   { subarray : array [] }, ] 

If your structure looks like that and you want to update subarray you have two options:

1. Change your structure so that you can leverage $pull.
2. Don't use $pull. Load the entire object into a client and use findAndModify.

Answer 2

MongoDB 3.6 added $[] operator that facilitates updates to arrays that contain embedded documents. So the problem can be solved by:

db.test.update(   { _id : "777" },   {$pull : {"someArray.$[].someNestedArray" : {"name":"delete me"}}} )