remove the 1000 first entries of the mongodb collection 'foo' by id

remove_n_entries.js

// count all the records first
db.getCollection('foo').find({}).count();

// save the the first 1000 entries ids to an array
removeIdsArray = db.getCollection('foo').find({})
    .limit(1000)
    .toArray()
    .map(function(doc) { return doc._id; });

// remove them by going through that array
db.getCollection('foo').remove({_id: {$in: removeIdsArray}});

// count the records again
db.getCollection('foo').find({}).count();