mongodb query with group()? - mongodb

Question

this is my collection structure :
coll{
id:...,
fieldA:{
fieldA1:[
{
...
}
],
fieldA2:[
{
text: "ciao",
},
{
text: "hello",
},
]
}
}
i want to extract all fieldA2 in my collection but if the fieldA2 is in two or more times i want show only one.
i try this
Db.runCommand({distinct:’coll’,key:’fieldA.fieldA2.text’})
but nothing. this return all filedA1 in the collection.
so i try
db.coll.group( {
key: { 'fieldA.fieldA2.text': 1 },
cond: { } },
reduce: function ( curr, result ) { },
initial: { }
} )
but this return an empty array...
How i can do this and see the execution time?? thank u very match...

Answer 1

Since you are running 2.0.4 (I recommend upgrading), you must run this through MR (I think, maybe there is a better way). Something like:
map = function(){
for(i in this.fieldA.fieldA2){
emit(this.fieldA.fieldA2[i].text, 1);
// emit per text value so that this will group unique text values
}
}
reduce = function(values){
// Now lets just do a simple count of how many times that text value was seen
var count = 0;
for (index in values) {
count += values[index];
}
return count;
}
Will then give you a collection of documents whereby _id is the unique text value from fieldA2 and the value field is of the amount of times is appeared i the collection.
Again this is a draft and is not tested.

Answer 2

I think the answer is simpler than a Map/Reduce .. if you just want distinct values plus execution time, the following should work:
var startTime = new Date()
var values = db.coll.distinct('fieldA.fieldA2.text');
var endTime = new Date();
print("Took " + (endTime - startTime) + " ms");
That would result in a values array with a list of distinct fieldA.fieldA2.text values:
[ "ciao", "hello", "yo", "sayonara" ]
And a reported execution time:
Took 2 ms