Mongo supports arrays of documents inside documents. For example, something like
{_id: 10, "coll": [1, 2, 3] }
Now, imagine I wanted to insert an arbitrary value at an arbitrary index
{_id: 10, "coll": [1, {name: 'new val'}, 2, 3] }
I know you can update values in place with $ and $set, but nothing for insertion. it kind of sucks to have to replace the entire array just for inserting at a specific index.
Starting with version 2.6 you finally can do this. You have to use $position operator. For your particular example:
db.students.update(
{ _id: 10},
{ $push: {
coll: {
$each: [ {name: 'new val'} ],
$position: 1
}
}}
)
The following will do the trick:
var insertPosition = 1;
var newItem = {name: 'new val'};
db.myCollection.find({_id: 10}).forEach(function(item)
{
item.coll = item.coll.slice(0, insertPosition).concat(newItem, item.coll.slice(insertPosition));
db.myCollection.save(item);
});
If the insertPosition is variable (i.e., you don't know exactly where you want to insert it, but you know you want to insert it after the item with name = "foo", just add a for() loop before the item.coll = assignment to find the insertPosition (and add 1 to it, since you want to insert it AFTER name = "foo".
Handy answer (not selected answer, but highest rated) from this similar post: Can you have mongo $push prepend instead of append?
utilizes $set to insert 3 at the first position in an array, called "array". Sample from related answer by Sergey Nikitin:
db.test.update({"_id" : ObjectId("513ad0f8afdfe1e6736e49eb")},
{'$set': {'array.-1': 3}})
Regarding your comment:
Well.. with concurrent users this is going to be problematic with any database...
What I would do is the following: Add a last modified timestamp to the document. Load the document, let the user modify it and use the timstamp as a filter when you update the document and also update the timestamp in one step. If it updates 0 documents you know it was modified in the meantime and you can ask the user to reload it.
Using the $position operator this can be done starting from version 2.5.3.
It must be used with the $each operator. From the documentation:
db.collection.update( <query>,
{ $push: {
<field>: {
$each: [ <value1>, <value2>, ... ],
$position: <num>
}
}
}
)
