mongodb - Check last element in array matches a condition -

i have array of numbers in mongodb documents , need check if last number in array meets conditions.

my documents stored this:

  {     name: string,     data: {       dates: array,       numbers: array     }   } 

and need check if last number in numbers "lies between" 2 other numbers.

any suggestions on how appreciated.

right effficient way have of doing using javascript evaluation of $where can find value of last array element , test programatically.

with sample documents:

{ "a": [1,2,3] }, { "a": [1,2,4] }, { "a": [1,2,5] } 

and query:

db.collection.find(function() { var = this.a.pop(); return ( > 2 ) & ( < 5 ) }) 

or presented $where string evaluation:

model.find(     {          "$where": "var = this.a.pop(); return ( > 2 ) && ( < 5 )"     },     function(err,results) {         // handling here     } ); 

which simple way , not have "overhead" such $unwind in aggregation framework created to "denormalize" , process arrays. not efficient there.

in "future" however, will be. available in development releases, there $slice operator aggregation framework. operator allow easy access "last" array element testing.

since aggregation framework operators in "native code" aand not javascript interpreted, single pipeline stage becomes more efficient javascript form. though listing looks longer in submission:

db.collection.aggregate([   { "$redact": {     "$cond": {       "if": {         "$anyelementtrue": {           "$map": {             "input": { "$slice": ["$a",-1] },             "as": "el",             "in":{               "$and": [                 { "$gt": [ "$$el", 2 ] },                 { "$lt": [ "$$el", 5 ] }               ]             }           }         }       },       "then": "$$keep",       "else": "$$prune"     }   }} ]) 

the $redact operator exists used "logically filter" comparison expression here. based on true/false match conditions either "keeps" or "prunes" document results repectively.

the $slice operator in it's aggregagtion framework form still untimately return array, albeit single element array in case. why $map used "transform" each element true/false condition , $anyelementtrue operator reduces "array" singular reponse repected $cond.

so when released, be efficient way this. until then, stick javascript presently fastest way to evaluation.

both query forms return first 2 documents of sample here:

{ "a": [1,2,3] }, { "a": [1,2,4] }