Auto increment / Sum one field of the query result? - mongodb

Question

I explain with an example:
I have a collection with 1 Million Items with ID: 123 each worth a different value:"worth"
user x with MONEY can "buy" the items. i basically want to know how many items the user can buy in a most elegant way.
so i got.
db.items.find({Item_ID:123},{Item_Age:1,Item_worth:1}.sort({Item_age:1})
-> gives me all Items with Item_ID:123 sorted by age.
I could now
Iterate through all items till Sum of Item Worth == User_Money but somehow i think this is not really efficient if the List returned matches 1 Million items and user might have only enough money for 1000
or
do the loop and query 1000 times
or
limit the query to 100 items. but this is very variably could still result in alot of loops
SO is there a query method which returns the sum of all values in each document??
Or any other efficient suggestions might be helpfull.
Thanks

Answer 1

Better than iterating would be do to this with mapReduce to get the running total for the items, and then filter the result.
So define a mapper as follows:
var mapper = function () {
totalWorth = totalWorth + this.Item_Worth;
var canBuy = userMoney >= totalWorth;
if ( canBuy ) {
emit(
{
Item_ID: this.Item_ID,
Item_Age: this.Item_Age
},
{
worth: this.Item_Worth,
totalWorth: totalWorth,
canBuy: canBuy
}
);
}
}
This accumulates a variable for totalWorth with the current "worth" of the item. Then a check is made to see the current totalWorth value exceeds the amount of userMoney that was input. If not, then you don't emit. Which is automatic filtering.
All the emitted keys are unique so just run the mapReduce as below:
db.items.mapReduce(
mapper,
function(){}, // reduce argument is required though not called
{
query: { Item_ID: 123 }
sort: { Item_ID: 1, Item_Age: 1 },
out: { inline: 1 },
scope: {
totalWorth: 0,
userMoney: 30
},
}
)
So looking at the other parts of that :
query: Is a standard query object you use to get your selection
sort: Is not required really because you are looking at Item_Age in ascending order. But if you wanted the oldest Item_Age first then you can reverse the sort.
out: Gives you an inline object as the result that you can use to get the matching items.
scope: Defines the global variables that can be accessed by the functions. So we provide an initial value for totalWorth and pass the parameter value of userMoney as how much money the user has to buy.
At the end of the day the result contains the filtered list of the items that fall under the amount of money the user can afford to purchase.