Finding documents that are valid on a specific date - mongodb

Question

I have some data stored in a mongodb collection similar to:
{"_id": 1, "category": "food", "name": "chips", "price": 1.50, "effectiveDate": ISODate("2013-03-01T07:00:00Z")}
{"_id": 2, "category": "food", "name": "chips", "price": 1.75, "effectiveDate": ISODate("2013-03-05T07:00:00Z")}
{"_id": 3, "category": "food", "name": "chips", "price": 1.90, "effectiveDate": ISODate("2013-03-10T07:00:00Z")}
{"_id": 4, "category": "beverage", "name": "pop", "price": 2.00, "effectiveDate": ISODate("2013-03-01T07:00:00Z")}
{"_id": 5, "category": "beverage", "name": "pop", "price": 2.25, "effectiveDate": ISODate("2013-03-05T07:00:00Z")}
{"_id": 6, "category": "beverage", "name": "pop", "price": 1.80, "effectiveDate": ISODate("2013-03-10T07:00:00Z")}
In mongodb, how would I go about writing a query that would return the documents that were active on a specific date, grouped by the category?
If I specified March 6, 2013 I'd expect to see the results:
{"_id": 2, "category": "food", "name": "chips", "price": 1.75, "effectiveDate": ISODate("2013-03-05T07:00:00Z")}
{"_id": 5, "category": "beverage", "name": "pop", "price": 2.25, "effectiveDate": ISODate("2013-03-05T07:00:00Z")}
I am new to mongo and have been trying to do this using group, aggregate and mapreduce but have been just spinning in circles.

Answer 1

To give you a real good answer, I need more details of your code and what you are trying to do. But if I understand right, I think you can solve that using only the aggregation framework. You should know that the aggregation framework uses the pipeline concept, in other words, the result of each step is used as the entry to the following.
My query:
db.yourcollection.aggregate([
/* First exclude everything that is superior to a given date */
{$match:{effectiveDate:{$lte:new Date(2013,2,6)}}},
/* Sort the rest by date, descending */
{$sort:{effectiveDate:-1}},
/* Group by name+category and grab only the first result
(the newest below that date) */
{$group:{_id:{name:'$name',category:'$category'}, effectiveDate:{$first:"$effectiveDate"},price:{$first:"$price"}}},
/* You said you want the results grouped by category.
This last $group does that and returns all matching products inside an array
It also removes the duplicates */
{$group:{_id:'$_id.category',products:{$addToSet:{name:"$_id.name",price:"$price",effectiveDate:"$effectiveDate"}}}}
])
The output is like this:
{
"result": [
{
"_id": "food",
"products": [
{
"name" : "chips",
"price" : 1.75,
"effectiveDate" : ISODate("2013-03-05T07:00:00Z")
}
]
},
{
"_id" : "beverage",
"products": [
{
"name" : "pop",
"price" : 2.25,
"effectiveDate" : ISODate("2013-03-05T07:00:00Z")
}
]
}
],
"ok":1
}
You can change the final output modifing the last $group or using a $project