$lookup support

Is new mongo 3.2 $lookup support planned?

[kofrasa]

Taking another look at this issue, it hit me that a collection abstraction is not necessary at all if we are willing to break a bit of MongoDB convention. The from field can refer directly to the join collection.

Example:

{
   $lookup:
     {
       from: <collection to join>, // this will be an Array
       localField: <field from the input documents>,
       foreignField: <field from the documents of the "from" collection>,
       as: <output_field> 
     }
}

In fact this is similar to what @justsml suggested above but will put the implementation of data sourcing outside of Mingo.

<strike>If a Function value is used for the from field, it will be called when the pipeline stage is reached to obtain the data to use. This is nice for a lazy effect and allows us to pass in values should we choose, which then can impact what the function will yield for the join. The latter part (params to function) is yet to be considered thoroughly.</strike>

Any comments?

[kofrasa]

Added in commit 6ae65098 Currently, the from field must be an array.