querying on date or time with mongoDB

Question

I have a collection with documents that have a start_time and end_time property. the strucute of the document is as follows:

{
_id     "4e9650ed1d41c811bc000000"
bid     31.0
start_time  2011-04-04 09:30:00 UTC
volume  31.0
end_time    2011-04-04 09:59:59 UTC
ask     0.0
price   1329.75
}

I want to be able to pull apart the date fields into date and time components and query on each seperately. An example query would be:

Get me all documents that are between (9:00 and 9:30) and between (2011-04-04 and 2011-05-04)

Answer 1

If space is not an issue to store additional denormalized fields and you are at the design part of the application (so you don't have to come back and modify existing data), you could:

• Store the start-hour and end-hour as additional attributes to keep the hour/minutes.
• Apply composite indices on start_time, start_hour and end_time, end_hour so your search performance is good.
• Run a query on all four fields.

See sample document:

{ 
  _id: "4e9650ed1d41c811bc000000", 
  bid: 31.0,
  start_time: 2011-04-04 09:30:00 UTC,
  start_hour: "09:30:00",  // new field
  volume: 31.0,
  end_time: 2011-04-04 09:59:59 UTC,
  end_hour: "09:59:59",    // new field
  ask: 0.0 ,
  price 1329.75 
}

It seems that the answer to your questions is that you cannot separate the components of the date and time efficiently in your query and for example take advantage of indices to speed up queries.

My suggestion above requires changing your schema slightly to achieve what you need. It seems that it would pay off greatly for the sacrifice of a few bytes extra on each document and the minor code changes.

If you have existing data, you could run a Map/Reduce to separate the components into a new collection and use that new collection going forward.

I hope this helps.