title: Mongoid on RoR3: 1) how to return specific field on query? 2) what inverse_of is needed for?
body: <p>Well, the title is self-explicative. But, let me elaborate a little better.
  First of all, I am using <strong>Mongoid</strong>, a Gem for using MongoDB with rails applications.</p>
  
  <p>#1) I have a large collection, which has information about a map. A map embeds a lot of tiles, and each tile references a terrain collection, a user collection, and has some other information. Therefore, if I obtain all tiles from a map, I will have a really large structure. However, I would like to cache a structure that comprises a matrix with only the terrain information. In order to do that, I select all tiles (and hence, all their unneeded information) and use only the terrain field. How can I select ONLY the terrain field on Mongoid? I tried operating with select on several ways, but I did not manage to do it.. (by the way, just for the sake of exemplification, I access the tiles array with the line "Map.first.tiles").</p>
  
  <p>#2) Well.. I am already here, so, why not ask this. Should I really use the inverse_of fields on my Models? I did not use it anywhere, and everything seems to be working perfectly. I do not see why it is needed, as it is pretty much straight forward to determine where to put them, and what they are the inverse of.</p>
  
  <p>Thanks in advance.
  Fernando.</p>