mongodb - Find document with closest integer value

问题:

Let's assume I have a collection with documents with a ratio attribute that is a floating point number.

{'ratio':1.437}

How do I write a query to find the single document with the closest value to a given integer without loading them all into memory using a driver and finding one with the smallest value of abs(x-ratio)?

回答1:

Interesting problem. I don't know if you can do it in a single query, but you can do it in two:

var x = 1; // given integer
closestBelow = db.test.find({ratio: {$lte: x}}).sort({ratio: -1}).limit(1);
closestAbove = db.test.find({ratio: {$gt: x}}).sort({ratio: 1}).limit(1);

Then you just check which of the two docs has the ratio closest to the target integer.

MongoDB 3.2 Update

The 3.2 release adds support for the $abs absolute value aggregation operator which now allows this to be done in a single aggregate query:

var x = 1;
db.test.aggregate([
    // Project a diff field that's the absolute difference along with the original doc.
    {$project: {diff: {$abs: {$subtract: [x, '$ratio']}}, doc: '$$ROOT'}},
    // Order the docs by diff
    {$sort: {diff: 1}},
    // Take the first one
    {$limit: 1}
])

回答2:

I have another idea, but very tricky and need to change your data structure.

You can use geolocation index which supported by mongodb

First, change your data to this structure and keep the second value with 0

{'ratio':[1.437, 0]}

Then you can use $near operator to find the the closest ratio value, and because the operator return a list sorted by distance with the integer you give, you have to use limit to get only the closest value.

db.places.find( { ratio : { $near : [50,0] } } ).limit(1)

If you don't want to do this, I think you can just use @JohnnyHK's answer :)