If you get the “$pullAll requires an array argument but was given a double” error in MongoDB, it’s because you didn’t provide an array as the value to replace.

Example

Suppose we have a collection with the following documents:

{ "_id" : 1, "bar" : [ 1, 7, 2, 3, 8, 7, 1 ] }
{ "_id" : 2, "bar" : [ 0, 1, 8, 17, 18, 8 ] }
{ "_id" : 3, "bar" : [ 15, 11, 8, 0, 1, 3 ] }

And we want to use $pullAll to replace all occurrences of a value in one of those documents.

Problem Code

Here’s an example of code that causes the above error:

db.foo.update( 
  { _id: 1 }, 
  { $pullAll: { bar: 7 } } 
)

Result:

WriteResult({
	"nMatched" : 0,
	"nUpserted" : 0,
	"nModified" : 0,
	"writeError" : {
		"code" : 2,
		"errmsg" : "$pullAll requires an array argument but was given a double"
	}
})

This happened because we forgot to surround the value 7 with square brackets. In other words, we forgot to provide an array.

Solution

Here’s the same code except this time we’ve provided the value as an array:

db.foo.update( 
  { _id: 1 }, 
  { $pullAll: { bar: [ 7 ] } } 
)

Result:

WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

This shows us that one document was matched and modified.

Let’s take a look at the collection now:

db.foo.find()

Result:

{ "_id" : 1, "bar" : [ 1, 2, 3, 8, 1 ] }
{ "_id" : 2, "bar" : [ 0, 1, 8, 17, 18, 8 ] }
{ "_id" : 3, "bar" : [ 15, 11, 8, 0, 1, 3 ] }

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