This note is in Hasura.GraphQL.Schema.Remote. It is referenced at:
When querying Remote schema interfaces, we need to re-construct
the incoming query to be compliant with the upstream remote.
We need to do this because the SelectionSet(s) that are
inputted to this function have the fragments (if any) flattened.
(Check flattenSelectionSet in ‘Hasura.GraphQL.Parser.Collect’ module)
The constructInterfaceSelectionSet function makes a valid interface query by:
Example: Suppose an interface ‘Character’ is defined in the upstream and two objects ‘Human’ and ‘Droid’ implement the ‘Character’ Interface.
Suppose, a field ‘hero’ returns ‘Character’.
{ hero { id name … on Droid { primaryFunction } … on Human { homePlanet } } }
When we parse the selection set of the hero field, we parse the selection set
twice: once for the Droid object type, which would be passed a selection set
containing the field(s) defined in the Droid object type and similarly once
for the ‘Human’ object type. The result of the interface selection set parsing
would then be the results of the parsing of the object types when passed their
corresponding flattened selection sets and the results of the parsing of the
interface fields.
After we parse the above GraphQL query, we get a selection set containing the interface fields and the selection sets of the objects that were queried in the GraphQL query. Since, we have the selection sets of the objects that were being queried, we can convert them into inline fragments resembling the original query and then query the remote schema with the newly constructed query.