在mongodb中使用$ in获取字段

Question

I have a collection and the example datas are like this,

{ L:"images", K:"asdd" } { L:"images", K:"asdd" } { L:"images", K:"asdd" } { L:"images", K:"asdd" } { L:"images", K:"asdd" } { L:"images", K:"asdd" } { L:"images", K:"asdd" } { L:"images", K:"asdd" } { L:"images", K:"asdd" } { L:"images", K:"asdd" } { L:"images", K:"asdd" } { L:"images", K:"asdd" } { L:"images", K:"asdd" }

and other fields are,{L:"cars",K:"asdff"},{L:"table",K:"asgeg"} those fields have at least 20 documents too, what I want as a result is like this

{ L:"images", K:"asdd" } { L:"images", K:"asdd" }

{ L:"cars", K:"asdd" } { L:"cars", K:"asdd" }

{ L:"table", K:"asdd" } { L:"table", K:"asdd" }

I want to get the documents according to their L field, but I want to limit every field's result to two and I have no idea how to manage this, thank you for any reply :)

Answer 1

Given your schema, I don't think there is a way, in a single query, to do what you want. I would recommend a PHP script with a simple foreach loop, or similar, to extract the values you want since you tagged this one as PHP.

Of course, you can just do it with 3 separate queries, one for each L value, with a limit of 2. I assume that we are really talking about a lot of distinct values and not just the three listed.

I'm not a PHP guy, so I can't help you out there, but you can use the JavaScript capabilities of the shell similarly like this (I loaded your sample data into a collection called foo and it had an _id field added automatically):

db.foo.distinct("L").forEach(function(key) {
    db.foo.find({"L" : key}, {_id : 0}).limit(2).forEach(
        function (value) {
            printjson(value);
        }
    )
})

I used distinct to generate the three values but you could easily just pass it in as an array on the first line like so:

['images', 'cars', 'table'].forEach(function(key) {

Either way, the function gave me the following output:

{ "L" : "images", "K" : "asdd" }
{ "L" : "images", "K" : "asdd" }
{ "L" : "cars", "K" : "asdd" }
{ "L" : "cars", "K" : "asdd" }
{ "L" : "table", "K" : "asdd" }
{ "L" : "table", "K" : "asdd" }