MongoDB query - if value appears in array x times

Question

{
  "_id": ObjectId("53ab1d2c256072374a5cc63f"),
  "title": "10% Off",
  "endDate": "2015-05-08",
  "limit" : "limited",
  "redemptions": [
    "1f7f5f96be3a",
    "kf40vksk03ps"
  ]
}
{
  "_id": ObjectId("53ab1d2c25607sfdgs74a5cc63f"),
  "title": "20% Off",
  "endDate": "2015-06-07",
  "limit" : "unlimited",
  "redemptions": [
    "1f7f5f96be3a",
    "1f7f5f96be3a",
    "kf40vksk03ps"
  ]
}

Story: a person can redeem a coupon 2 times. After 2 times, don't return it. How can I check that a value appears less than 2 times??

Wish it was as easy as:

{ 'redemptions' : { $exists: true }, $where : 'this.redemptions.$.1f7f5f96be3a.length < 2' }

How can I get a count for how many times a specific value is in an array and compare on that?

Edit So to add some fun. I updated my schema, so I need to put that into a conditional. If limit = 'unlimited' { return record } if limit = 'limited' { return only if array has less than 2 values = '1f7f5f96be3a'

Christian P · Accepted Answer · 2014-06-25 22:16:47Z

2

You can do what you want with aggregation framework:

db.collection.aggregate([
    /* find only documents that have redemption key */
    { $match : { redemptions : { $exists: true }}},
    /* unwind the redemptions array so we can filter documents by coupons */
    { $unwind : "$redemptions" },
    /* filter only coupons you're looking for */
    { $match : { redemptions : "1f7f5f96be3a"}},
    /* group the documents back so we can calculate the count */
    { $group : { _id : "$_id",
                 title : { $first : "$title" },
                 endDate : { $first : "$endDate" },
                 count :  {$sum : 1 }}},
    /* finally, count the documents that have less than 2 coupons */
    { $match : { count : { $lt :2 }}}
]);

Edit:

You just need to change the $group and last $match stages:

{ $group : { _id : "$_id",
            title : { $first : "$title" },
            endDate : { $first : "$endDate" },
            limit : { $first : "$limit" },
            count :  {$sum : 1 }}},
/* finally, count the documents that have less than 2 coupons 
   or have limit "unlimited */
{ $match : { $or : [{ count : { $lt :2 }}, {limit : "unlimited"}]}}

edited Jun 25, 2014 at 22:16

answered Jun 25, 2014 at 21:34

Christian P

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1 Comment

user3776686 Over a year ago

Thanks so much, got me much closer. So to add some fun... I updated my schema, so I need to put that into a conditional. if limit = 'unlimited' { return record } if limit = 'limited' { return only if array has less than 2 values = '1f7f5f96be3a' Anyone of doing that. Basically what you did but in a conditional?

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