How to traverse all vertex and get nested objects - gremlin

Question

I want to get nested objects in the form of
{ country :
{code:'IN',states:
{code:'TG',cities:
{code:'HYD',malls:
{[shopping-mall1],[shopping-mall2],.....}
},
{code:'PKL',malls:
{[shopping-mall1],[shopping-mall2],.....}
}
},
{code:'AP',cities:
{code:'VJY',malls:
{[shopping-mall1],[shopping-mall2],.....}
}
}
}
}
MY graph is in format
vertex: country ---> states ---->cities ---> mallls
edges: (type:'state') ('type','city')
ex: inE('typeOf').outV().has('type','state') move to next vertex "states".
next same inE('typeOf').outV().has('type','city') moves to "city" vertex. then "malls" vertex .
And tired to write the code, some vertex has no cities i have an error that situavation."
error
The provided traverser does not map to a value: v[8320]->[JanusGraphVertexStep(IN,[partOf],vertex), HasStep([type.eq(city)]), JanusGraphPropertiesStep([code],value)]
Thats why i am using coalesce because some state has not an edge 'inE('partOf').outV().has('type','city')' means no city
.by(coalesce(select('states').inE('partOf').outV().has('type','city'))
My query
g.V().hasLabel('Country').has('code','IN')
.project('country')
.by(project('code','states')
.by(values('code'))
.by(inE('partOf').outV().has('type','state').has('code').as('states').
project('code','cities')
.by(select('states').values('code'))
.by(coalesce(select('states').inE('partOf').outV().
has('type','city').has('code').as('cities').
project('code','malls')
.by(select('cities').values('code'))
.by(coalesce(select('cities').inE('partOf').outV().
has('type','malls').valueMap(),constant(0))),
constant(0)))))
But the result is
{country={code=IN, states={code=DD, cities=0}}}
here i am getting one state 'DD' and that state is no city,so it gives 'cities = 0".
the above result is only one state is coming, i want all states, cities and malls in each city.
Please update query or change query

Answer 1

In order to collect all the results you should use .fold() traversal which returns a list of the collected traversals. without fold you will get only the first traversal like in your example.
In order to keep the types the same I changed the constant to [] instead of 0.
It was also not clear if the "type" property is on the edge or the vertex. I find it more appropriate to have it on the edge, so I fixed it as well by moving the has('type',...) between the inE() and outV().
Last, you don't need to "store" the traversal using "as" and then "select" it.
This query should give you the required result:
g.V().hasLabel('Country').has('code','IN')
.project('country')
.by(project('code','states')
.by(values('code'))
.by(inE('partOf').has('type','state').outV().has('code')
.project('code','cities')
.by(values('code'))
.by(coalesce(inE('partOf').has('type','city').outV().has('code')
.project('code','malls')
.by(values('code'))
.by(coalesce(
inE('partOf').has('type','malls').outV().valueMap(),
constant([])).fold()),
constant([])).fold())
.fold()))