MongoDB - Mongoose find command to get only values from an embedded object - node.js

Question

My Schema looks something like this.
{
_id: '1',
items: {
'id1': 'item1',
'id2': 'item2',
'id3': 'item3'
}
}
Following is the query
ItemModel.find({}, {
items: 1,
_id: 0
});
And the result of the find query is:
{ "items" : { "21" : "item21", "22" : "item22", "23" : "item23" } }
{ "items" : { "31" : "item31", "32" : "item32", "33" : "item33" } }
{ "items" : { "11" : "item11", "12" : "item32", "13" : "item13" } }
What I want is:
["item21", "item22", "item23",
"item31", "item32", "item33",
"item11", "item12", "item13"]
Currently, I am doing the processing on the node.js end for getting the above. I want to reduce the output payload size coming from MongoDB. The "items" key is redundant and the IDs mentioned are not required as well when I fetch it. Here, the IDs are quite small like 21, 22, 13, etc. but those are acutally 50 characters in length.
If not the above, any other efficient alternatives are also welcome.

Answer 1

One example of how to achieve that is the following aggregation:
[
{
$project: {
items: {
$objectToArray: '$items',
},
},
},
{ $unwind: '$items' },
{
$project: {
items: '$items.v',
},
},
{
$group: {
_id: null,
items: {
$push: '$items',
},
},
}
];
What this does is first we convert with $project & $objectToArray field to an array so that we could use $unwind. This way we'll have documents with different items. Now we convert with another $project to make it a string instead of an object (which would be { v: <value>, k: <value> }. And, finally, we $group them together.
Final result:
To get exactly that list, you'll need in your code to access items field, like result[0].items ([0] because aggregation will return an array).