neo4j shortest path works only when no nodes are specified - neo4j

Question

I am trying to obtain the node and edge ids for the shortest path between two nodes in my neo4j graph database.
If I do not specify which nodes I want, the code runs somehow and returns a path:
import py2neo
graph.run("MATCH (start:Point)-[:SOURCE_POINT]->(r:Road)-[:TARGET_POINT]->(end:Point) \
CALL apoc.algo.dijkstraWithDefaultWeight(start, end, 'Road', 'length', 10.0) \
YIELD path as path, weight as weight \
UNWIND nodes(path) as n \
RETURN DISTINCT { id : id(n), labels : labels(n), data: n} as node").to_table()
But when I run the same code and specify which nodes I want, it returns empty:
graph.run("MATCH (start:Point {id: '4984061949'})-[:SOURCE_POINT]->(r:Road)-[:TARGET_POINT]->(end:Point {id: '4984061963'}) \
...
If I simply try to match those node id's, it returns them ok - so I know they are in the db.
I'm thinking it could be because my 'cost' is a string. But I'm not sure how to cast it to float before it goes through the dijkstraWithDefaultWeight function.

Answer 1

You seem to have a couple of issues.
1. MATCH clause is too restrictive
The following MATCH clause would only succeed if there was a path between the specified start and end nodes consisting of just one Road:
MATCH (start:Point {id: '4984061949'})-[:SOURCE_POINT]->(r:Road)-[:TARGET_POINT]->(end:Point {id: '4984061963'})
If that MATCH clause fails, then your query would return nothing.
The following MATCH clause would succeed if there was a path between any pair of Point nodes consisting of one Road:
MATCH (start:Point)-[:SOURCE_POINT]->(r:Road)-[:TARGET_POINT]->(end:Point)
If that MATCH clause succeeds, then, of course, the Dijkstra procedure will also succeed.
Instead of either of the above, you should probably just use MATCH to get the two endpoints and let the Dijkstra algorithm do the job of finding the path:
MATCH (start:Point {id: '4984061949'}), (end:Point {id: '4984061963'})
2. Wrong procedure argument(s)
The third argument passed to apoc.algo.dijkstraWithDefaultWeight is supposed to specify relationship types and directions, not node labels. Also, the last 2 arguments are supposed to be a relationship property and default relationship property value, respectively.