Mongodb aggregate match query with priority on full match - mongodb

Question

I am attempting to do a mongodb regex query on a field. I'd like the query to prioritize a full match if it finds one and then partials afterwards.
For instance if I have a database full of the following entries.
{
"username": "patrick"
},
{
"username": "robert"
},
{
"username": "patrice"
},
{
"username": "pat"
},
{
"username": "patter"
},
{
"username": "john_patrick"
}
And I query for the username 'pat' I'd like to get back the results with the direct match first, followed by the partials. So the results would be ordered ['pat', 'patrick', 'patrice', 'patter', 'john_patrick'].
Is it possible to do this with a mongo query alone? If so could someone point me towards a resource detailing how to accomplish it?
Here is the query that I am attempting to use to perform this.
db.accounts.aggregate({ $match :
{
$or : [
{ "usernameLowercase" : "pat" },
{ "usernameLowercase" : { $regex : "pat" } }
]
} })

Answer 1

Given your precise example, this could be accomplished in the following way - if your real world scenario is a little bit more complex you may hit problems, though:
db.accounts.aggregate([{
$match: {
"username": /pat/i // find all documents that somehow match "pat" in a case-insensitive fashion
}
}, {
$addFields: {
"exact": {
$eq: [ "$username", "pat" ] // add a field that indicates if a document matches exactly
},
"startswith": {
$eq: [ { $substr: [ "$username", 0, 3 ] }, "pat" ] // add a field that indicates if a document matches at the start
}
}
}, {
$sort: {
"exact": -1, // sort by our primary temporary field
"startswith": -1 // sort by our seconday temporary
}
}, {
$project: {
"exact": 0, // get rid of the "exact" field,
"startswith": 0 // same for "startswith"
}
}])
Another way would be using $facet which may prove a bit more powerful by enabling more complex scenarios but slower (several people here will hate me, though, for this proposal):
db.accounts.aggregate([{
$facet: { // run two pipelines against all documents
"exact": [{ // this one will capture all exact matches
$match: {
"username": "pat"
}
}],
"others": [{ // this one will capture all others
$match: {
"username": { $ne: "pat", $regex: /pat/i }
}
}]
}
}, {
$project: {
"result": { // merge the two arrays
$concatArrays: [ "$exact", "$others" ]
}
}
}, {
$unwind: "$result" // flatten the resulting array into separate documents
}, {
$replaceRoot: { // restore the original document structure
"newRoot": "$result"
}
}])