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Projection generator for mongodb
_ = 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
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 '', ''
# => [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')
{ $match: email: $in: emails }
{ $project: projection }
{ $sort: weight: 1 }
users = ['lmj112', 'amv1882', 'thb12']
console.log(JSON.stringify (buildProjection users, 'login'), undefined, 2)
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