如何使用java检查MongoDB中的字符串数组是否为空？

Question

Imagine I have a DB with documents with the format:

{
  "id": "12345",
  "address": ["adress1","adress2","adress3"]

}

Now imagining that can be cases where one id has no addresses

{
  "id": "12346",
  "address": [""]

}

I have tried, after query the DB and get the document of the id I want, .isEmpty(), but it returns false.

Can this be done using MongoDB?

Answer 1

If you have to deal with blank addresses ( "address": [""] is different than "address": [] which is different than "address": null which is different than not setting the address field at all, then you have to query like thisL

db.foo.aggregate([
{$addFields: {X: {$reduce: {
                input: "$address",
                initialValue: 0,
                in: {$sum: [ "$$value", {"$cond":[ {"$ne": ["$$this",""]}, 1, 0]} \
]}
            }}
    }}
,{$match: {"X": {$gt:0} }}
                       ]);

We use the $reduce function to "walk" the address array and check each item to see if it is not a blank string. If not blank, we increment the $$value field which at the end is assigned to new field X . If the array is not present or contains only blank addresses (one or more) then X will be zero and we filter those out with the subsequent $match .