mongodb - How to check if a portion of an _id from one collection appears in another -

i have collection _id of form [message_code]-[language_code] , _id [message_code]. i'd find documents first collection message_code portion of _id not appear in second collection.

example:

> db.cola.find({}) { "_id" : "trm1-en" } { "_id" : "trm1-es" } { "_id" : "trm2-en" } { "_id" : "trm2-es" } > db.colb.find({}) { "_id" : "trm1" } 

i want query return trm2-en , trm-es cola. of course in live data, there thousands of records in each collection.

according this question trying similar, have save results query against colb , use in $in condition in query against cola. in case, need strip -[language_code] portion before doing comparison, can't find way so.

if else fails, i'll create new field in cola contains message code, there better way it?

edit: based on michael's answer, able come solution:

var arr = db.colb.distinct("_id") var regexs = arr.map(function(elm){    return new regexp(elm); }) var result = db.cola.find({_id : {$nin : regexs}}, {_id : true}) 

edit: upon closer inspection, above method doesn't work after all. in end, had add new field.

disclaimer: little hack may not end well.

• get distinct _id using collection.distinct method.
• build regular expression array using array.prototype.map()

var arr = db.colb.distinct('_id'); arr.map(function(elm, inx, tab) {     tab[inx] = new regexp(elm);  });  db.cola.find({ '_id': { '$nin': arr }})