popcorn :: Repository :: find_by_id
idid is an abstract thing at this stage
TODO maybe introduce the PrimaryKey concept?
	# Find an instance by it's `id`
	#
	# `id` is an abstract thing at this stage
	# TODO maybe introduce the `PrimaryKey` concept?
	fun find_by_id(id: String): nullable E is abstract
					lib/popcorn/pop_repos.nit:172,2--176,51
				
	redef fun find_by_id(id) do
		var query = new JsonObject
		query["_id"] = id
		return find(query)
	end
					lib/popcorn/pop_repos.nit:317,2--321,4