Combining Graph Traversals

Finding the start vertex via a geo query

Our first example will locate the start vertex for a graph traversal via a geo index. We use the city graph and its geo indices:

```arangosh> var examples = require("@arangodb/graph-examples/example-graph.js");
arangosh> var g = examples.loadGraph("routeplanner");
```

We search all german cities in a range of 400 km around the ex-capital Bonn: Hamburg and Cologne. We won't find Paris since its in the `frenchCity` collection.

Query:
```  FOR startCity IN germanCity
FILTER GEO_DISTANCE(@bonn, startCity.geometry) < @radius
RETURN startCity._key
```
Bind Values:
```{
"bonn": [
7.0998,
50.734
],
}
```
Query results:
```[
"Cologne",
"Hamburg"
]```

Lets revalidate that the geo indices are actually used:

Explain Query:
```  FOR startCity IN germanCity
FILTER GEO_DISTANCE(@bonn, startCity.geometry) < @radius
RETURN startCity._key
```
Bind Values:
```{
"bonn": [
7.0998,
50.734
],
}
```

And now combine this with a graph traversal:

Query:
```  FOR startCity IN germanCity
FILTER GEO_DISTANCE(@bonn, startCity.geometry) < @radius
FOR v, e, p IN 1..1 OUTBOUND startCity
GRAPH 'routeplanner'
RETURN {startcity: startCity._key, traversedCity: v._key}
```
Bind Values:
```{
"bonn": [
7.0998,
50.734
],
}
```

The geo index query returns us `startCity` (Cologne and Hamburg) which we then use as starting point for our graph traversal. For simplicity we only return their direct neighbours. We format the return result so we can see from which `startCity` the traversal came.

Alternatively we could use a `LET` statement with a subquery to group the traversals by their `startCity` efficiently:

Query:
```  FOR startCity IN germanCity
FILTER GEO_DISTANCE(@bonn, startCity.geometry) < @radius
LET oneCity = (
FOR v, e, p IN 1..1 OUTBOUND startCity
GRAPH 'routeplanner' RETURN v._key
)
RETURN {startCity: startCity._key, connectedCities: oneCity}
```
Bind Values:
```{
"bonn": [
7.0998,
50.734
],
```arangosh> examples.dropGraph("routeplanner");