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.

---

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.