Sort documents based on first character in field value - regex

Question

I have a set of data like this:
[{name: "ROBERT"}, {name: "PETER"}, {name: "ROBINSON"} , {name: "ABIGAIL"}]
I want to make a single mongodb query that can find:
Any data which name starts with letter "R" (regex: ^R)
Followed by any data which name contains letter "R" NOT AS THE FIRST CHARACTER, like: peteR, adleR, or caRl
so it produces:
[{name: "ROBERT"}, {name: "ROBINSON"}, {name: "PETER"}]
it basically just display any data that contains "R" character in it but I want to sort it so that data with "R" as the first character appears before the rest
So far I've come out with 2 separate query then followed by an operation to eliminate any duplicated results, then joined them. So is there any more efficient way to do this in mongo ?

Answer 1

What you want is add a weight to you documents and sort them accordingly.
First you need to select only those documents that $match your criteria using regular expressions.
To do that, you need to $project your documents and add the "weight" based on the value of the first character of your string using a logical $condition processing.
The condition here is $eq which add weight 1 to the document if the lowercase of the first character in the name is "r" or 0 if it's not.
Of course the $substr and the $toLower string aggregation operators respectively return the the first character in lowercase.
Finally you $sort your documents by weight in descending order.
db.coll.aggregate(
[
{ "$match": { "name": /R/i } },
{ "$project": {
"_id": 0,
"name": 1,
"w": {
"$cond": [
{ "$eq": [
{ "$substr": [ { "$toLower": "$name" }, 0, 1 ] },
"r"
]},
1,
0
]
}
}},
{ "$sort": { "w": -1 } }
]
)
which produces:
{ "name" : "ROBERT", "w" : 1 }
{ "name" : "ROBINSON", "w" : 1 }
{ "name" : "PETER", "w" : 0 }

Answer 2

try this :
db.collectioname.find ("name":/R/)