Push object into array if the array exists otherwise create the array with object in MongoDB

Question

I have collection with a document like this:

{
    _id : \"abcd\",
    name : \"Tom\",
    myArray : [
        {
            field1 : \"\",
            field         


        

Answer 1

To explain all the possible cases here, the consider each document case:

If your document to alter looks like this:

{
    "_id": "efgh",
    "name": "Jerry"
}

Then an update statment like this:

db.collection.update(
    { "_id": "efgh" },
    { "$push": { "myArray": { "field1": "abc", "field2": "def" } } }
)

Results in this:

{
    "_id": "efgh",
    "name": "Jerry",
    "myArray": [
        {
            "field1": "abc",
            "field2": "def"
        }
    ]
}

So the array is created and the new item appended.

If your document already has an array like this:

{
    "_id": "abcd",
    "name": "Tom",
    "myArray": [
        {
            "field1": "",
            "field2": ""
        }
    ]
}

And you do basically the same statement:

db.collection.update(
    { "_id": "abcd" },
    { "$push": { "myArray": { "field1": "abc", "field2": "def" } } }
)

Then the new document content is appended to the existing array:

{
    "_id": "abcd",
    "name": "Tom",
    "myArray": [
        {
            "field1": "",
            "field2": ""
        },
        {
            "field1": "abc",
            "field2": "def"
        }
    ]
}

If however your original document has the named field but it is not an array, like this:

{
    "_id": "efgh",
    "name": "Jerry",
    "myArray": 123
}

Then make sure it is not an array by testing in the query condtion and using $set instead:

db.collection.update(
    { "_id": "efgh", "myArray.0": { "$exists": false } },
    { "$set": { "myArray": [{ "field1": "abc", "field2": "def" }] } }
)

That will safely overwrite the element that is not an array ( dot notation "myArray.0" means first array element, which is not true ) with a new array containing your content. The result is the same as the original:

{
    "_id": "efgh",
    "name": "Jerry",
    "myArray": [
        {
            "field1": "abc",
            "field2": "def"
        }
    ]
}