# A relationship between two nodes.
# Relationships between nodes are a key part of a graph database.
# They allow for finding related data. Just like nodes, relationships can have properties.
#
# Create a relationship:
#
#     var client = new Neo4jClient("http://neo4j:7474")
#     # Create nodes
#     var andres = new NeoNode
#     andres["name"] = "Andres"
#     var kate = new NeoNode
#     kate["name"] = "Kate"
#     # Create a relationship of type `LOVES`
#     var loves = new NeoEdge(andres, "LOVES", kate)
#     client.save_edge(loves)
#     assert loves.is_linked
#
# Get an edge from DB:
#
#     var url = loves.url.to_s
#     var edge = client.load_edge(url)
#     assert edge.from["name"].to_s == "Andres"
#     assert edge.to["name"].to_s == "Kate"
class NeoEdge
	super NeoEntity

	private var internal_from: nullable NeoNode
	private var internal_to: nullable NeoNode
	private var internal_type: nullable String
	private var internal_from_url: nullable String
	private var internal_to_url: nullable String

	init(from: NeoNode, rel_type: String, to: NeoNode) do
		self.internal_from = from
		self.internal_to = to
		self.internal_type = rel_type
	end

	redef init from_neo(neo, url) do
		super
		var obj = neo.get(url).as(JsonObject)
		self.internal_type = obj["type"].to_s
		self.internal_from_url = obj["start"].to_s
		self.internal_to_url = obj["end"].to_s
	end

	redef init from_json(neo, obj) do
		super
		self.internal_type = obj["type"].to_s
		self.internal_from_url = obj["start"].to_s
		self.internal_to_url = obj["end"].to_s
	end

	# Get `from` node
	fun from: NeoNode do return internal_from or else load_from

	private fun load_from: NeoNode do
		var node = neo.load_node(internal_from_url.to_s)
		internal_from = node
		return node
	end

	# Get `to` node
	fun to: NeoNode do return internal_to or else load_to

	private fun load_to: NeoNode do
		var node = neo.load_node(internal_to_url.to_s)
		internal_to = node
		return node
	end

	# Get edge type
	fun rel_type: nullable String do return internal_type

	# Get the JSON body of a REST request that create the relationship.
	private fun to_rest: JsonObject do
		var obj = new JsonObject
		if to.is_linked then
			obj["to"] = to.url
		else
			obj["to"] = "\{{to.batch_id.to_s}\}"
		end
		obj["type"] = rel_type
		obj["data"] = properties
		return obj
	end
end