mongo $push overwrites rather than adds to an array within a document - node.js

Question

I am trying to create a historical record for updates to a document in Mongo DB via NodeJS. The document updates are only in one object within the document, so it seems like creating an array of historical values makes sense.
However, when I use the $push function with db.collection.update(), it only updates the array at the 0 index rather than add to the array.
Here is what I have:
{
_id: ID,
odds: {
spread: CURRENTSPREAD,
total: CURRENTTOTAL,
history: [
0: {
spread: PREVIOUSSPREAD1,
total: PREVIOUSTOTAL1,
date: DATEENTERED
}
]
}
}
Here is what I would like:
{
_id: ID,
odds: {
spread: CURRENTSPREAD,
total: CURRENTTOTAL,
history: [
0: {
spread: PREVIOUSSPREAD1,
total: PREVIOUSTOTAL1,
date: DATEENTERED1
},
1: {
spread: PREVIOUSSPREAD2,
total: PREVIOUSTOTAL2,
date: DATEENTERED2
},
...,
n: {
spread: PREVIOUSSPREAD-N,
total: PREVIOUSTOTAL-N,
date: DATEENTERED-N
}
]
}
}
There is no need to check whether the previous value exists before adding.
Here is my code:
var oddsHistoryUpdate = {
$push: {
'odds.history': {
spread: game.odds.spread,
total: game.odds.total,
date: Date.now()
}
}
}
db.collection('games').update({"_id": ID}, oddsHistoryUpdate).
.then(finish executing)
Why is it only pushing to the 0 index instead of adding to the array? How do I fix?

Answer 1

Bigga_HD's answer is the correct one regarding the $push operator. However, there may be an alternative solution that is more aligned to how MongoDB works under the hood.
A single document in MongoDB has a hard limit of 16MB, and if a document is frequently updated, it is possible that the array grows so large that it hits this limit.
Alternatively, you can just insert a new document into the collection instead of pushing the old document inside an array. The new & old documents can be differentiated by their insertion date. For example:
{
_id: ID,
name: <some identification>
insert_date: ISODate(...),
odds: {
spread: CURRENTSPREAD,
total: CURRENTTOTAL
}
}
You can then query the collection using a combination of e.g. its name and insert_date, sorted by its date descending, and limit by 1 to get the latest version:
db.collection.find({name: ...}).sort({insert_date: -1}).limit(1)
or remove the limit to find all versions:
db.collection.find({name: ...}).sort({insert_date: -1})
To support this query, you can create an index based on name and insert_date in descending order (see Create Indexes to Support Your Queries)
db.collection.createIndex({name: 1, insert_date: -1})
As a bonus, you can use a TTL index on the insert_date field to automatically delete old document versions.

Answer 2

$push
The $push operator appends a specified value to an array.
The $push operator has the form:
{ $push: { <field1>: <value1>, ... } }
If the field is absent in the document to update, $push adds the array field with the value as its element.
If the field is not an array, the operation will fail.
If the value is an array, $push appends the whole array as a single element. To add each element of the value separately, use the $each modifier with $push.
$each -Appends multiple values to the array field.
This should do the trick for you. Obviously, it's a very simplified example.
{ $push: { <field1>: { <modifier1>: <value1>, ... }, ... } }
let oddsHistoryUpdate = {
spread: game.odds.spread,
total: game.odds.total,
date: Date.now()
}
db.games.update(
{ _id: ID },
{ $push: { odds.history: oddsHistoryUpdate} }
)
I suggest try using Mongoose for your NodeJS - MongoDB interactions.

Answer 3

The answer was uncovered by dnickless.
In a previous call, I update the main odds object which I didn't realize was wiping out the history array.
Updating the previous call from
update($set: {odds: { spread: SPREAD, total: TOTAL }})
to
update($set: {"odds.spread": SPREAD, "odds.total": TOTAL})
and then making my $push call as written, all works fine.