Projection generator for mongodb

agg.coffee

_ = require 'underscore'

# Given an array of elements ELEM and a matching KEY value,
# will build the apprpriate projection to generate sortable
# weights for a mongo aggregator.
#
#   ELEM: An array of values upon which to match against KEY
#   KEY:  The document field key to match against
#   I:    Default 0, index into array at which to begin
#
# Will return formatted projections. Note that elems must
# contain at least two elements for the initial call, else
# only the initial I value will be returned.
buildProjection = (elems, key, i = 0) ->
  return i if (elems.length - i) == 1
  $cond:
    if: { $eq: [ "$#{key}", elems[i] ] }
    then: i
    else: buildProjection elems, key, i+1

# Assuming we have a mongoose userSchema, with the standard
# schema paths defined on userSchema.paths, we can make use
# of that objects keys to prevent $project from masking fields
# and then build our weights againsts the email key.
#
# Example.
#
#   exampleQuery 'lmj112@ic.ac.uk', 'amv1882@ic.ac.uk'
#   => [User(lmj112), User(amv1882)]
#
# Calling exec() simply generates a promise.
exampleQuery = (emails...) ->
  projection =
    _.object ([k,1] for own k,v of userSchema.paths)
  projection.weight = buildProjection(emails, 'email')
  User.aggregate([
    { $match: email: $in: emails }
    { $project: projection }
    { $sort: weight: 1 }
  ]).exec()

users = ['lmj112', 'amv1882', 'thb12']

console.log(JSON.stringify (buildProjection users, 'login'), undefined, 2)