Field Selection
Field selection occurs when you have a compound type (an object or interface type) and you select a set of sub fields from that type.
For example given the following query :
query {
user(userId : "xyz") {
name
age
weight
friends {
name
}
}
}
The field selection set of the user
field is name
, age
, weight
, friends
and friends/name
Knowing the field selection set can help make DataFetcher
s more efficient. For example in the above query
imagine that the user
field is backed by an SQL database system. The data fetcher could look ahead into the field selection
set and use different queries because it knows the caller wants friend information as well as user information.
graphql.schema.DataFetchingFieldSelectionSet
is used to represent this field selection set. It gives you maps
of the fields and their graphql.schema.GraphQLFieldDefinition
s and argument values.
DataFetcher smartUserDF = new DataFetcher() {
@Override
public Object get(DataFetchingEnvironment env) {
String userId = env.getArgument("userId");
DataFetchingFieldSelectionSet selectionSet = env.getSelectionSet();
if (selectionSet.contains("friends/*")) {
return getUserAndTheirFriends(userId);
} else {
return getUser(userId);
}
}
};
A glob path matching system is used for addressing fields in the selection. Its based on java.nio.file.FileSystem#getPathMatcher
as an implementation.
This will allow you to use *
, **
and ?
as special matching characters such that invoice/customer*
would
match an invoice
field with child fields that start with customer
. Each level of field is separated by /
just like
a file system path.
There are methods that allow you to get more detailed information about the fields in the selection set. For example
if you are using Relay often you want to know what fields have
been request in the Connection
section of the query.
So given a query like:
query {
users(first:10) {
edges {
node {
name
age
weight
friends {
name
}
}
}
}
}
you can write code that gets the details of each specific field that matches a glob.
DataFetchingFieldSelectionSet selectionSet = env.getSelectionSet();
List<SelectedField> nodeFields = selectionSet.getFields("edges/nodes/*");
nodeFields.forEach(selectedField -> {
System.out.println(selectedField.getName());
System.out.println(selectedField.getFieldDefinition().getType());
DataFetchingFieldSelectionSet innerSelectionSet = selectedField.getSelectionSet();
// this forms a tree of selection and you can get very fancy with it
}