class User
  include Mongoid::Document

  has_many :cgm_evgs
end

class CgmEvg
  include Mongoid::Document

  field :timestamp, type: Date

  belongs_to :user
end

User.find(_id: u1.id).cgm_evgs.min(:timestamp).to_a

{
	"aggregate":"cgm_evgs",
	"pipeline":[
		{"$match":{"user_id":{"$oid":"65a7dfcfe61d72729750604a"},"timestamp":{"$exists":true}}},
		{
			"$group":{
				"_id":"timestamp",
				"count":{"$sum":1},
				"max":{"$max":"$timestamp"},
				"min":{"$min":"$timestamp"},
				"sum":{"$sum":"$timestamp"},
				"avg":{"$avg":"$timestamp"}
			}
		}
	]
}