Here is the rake task that will simulate error. Error appears only when validates line is present.

class TestCollection
include Mongoid::Document
include Mongoid::Timestamps

field :text, type: String

validates :text, presence: true
end

namespace :error do
desc "Simulate error in mongoid 7.0.0"
task :simulate => :environment do
test = TestCollection.new(
text: 'text'
)
test.save
end
end

by
TheR