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 : "",
            field2 : ""
        }
    ]
},
{
    _id : "efgh",
    name : "Jerry"
}

I have a new object for myArray. I want to write a query to update only one document.

If the query matches the document with _id : "abcd", then push the new object in to myArray field:

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

And if the query matches with _id : "efgh" create the field myArray with new object inside it:

{
    _id : "efgh",
    name : "Jerry"
    myArray : [
         {
              // new object
         }
    ]
}

How can I achieve this?

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"
        }
    ]
}

Answer 2

$Push is what you need to achieve this. If the myArray is already existing then $push will insert the newObject to it( similar to the STACK data structure operation ). If the myArray doesn't exist, it will create the field "myArray" as key and will insert the newObject into it ( similar to $Set oprtaion )

Consider another below sample document :

{
_id : "abcd"
myArray:[
{
  field1:"mango",
  field2:"apple"
}
       ]
}

so now lets Insert the below object to it :

var newObject = {field1:"example1" , field2:"exmaple2"};

Query :

db.collection.update({_id:"abcd"},{"$push":{myArray: newObject }});

after execution of the above query, result is as below :

{
    _id : "abcd"
    myArray:[
    {
      field1:"mango",
      field2:"apple"
    },
    {
      field1:"example1" , 
      field2:"exmaple2"
    }
           ]
    }

Now let us consider another example where "myArray" key is NOT PRESENT in the document :

{
_id:"efg"
}

so lets insert the below object :

 var newObject2 = { field1 : "exp2" , field2 : "exp1" }

Query :

 db.collection.update({_id:"efg"},{"$push":{myArray: newObject2 }});

Result of the above UPDATE operation is below :

 {
   _id : "efg"
   myArray : [ 
   { 
    field1 : "exp2" , 
    field2 : "exp1"
  }
          ]
}