FieldSelection Extension

The FieldSelection extension adds an enhanced version of the Attributor::FieldSelector type, Praxis::Extensions::FieldSelection::FieldSelector (also aliased under Praxis::Types::FieldSelector). This may be used both generically (i.e., directly as FieldSelector), or scoped to a specific MediaType with FieldSelector.for so that it can perform validation.

After parsing, the selected set of fields is available as the fields accessor on the parameter.

For example, to define a parameter to select a set of fields from a Person media type, you would define a :fields attribute in the params like: attribute :fields, Praxis::Types::FieldSelector.for(Person). The parsed fields in the request would then be available with request.params.fields.fields.

To use this extension, use require 'praxis/extensions/field_selection'.

Field Selection Syntax

The field selection syntax is a simple comma-separated list of attributes to select, with sub-attributes being selectable by enclosing them in curly-braces (“{}”). I.e., compatible with the basic selection hierarchy of GraphQL.

This is parsed by FieldSelector and translated to an equivalent nested hash, with field names as keys, and values of either true to indicate that field was selected or a sub-hash with additional sub-fields for selection.


  • a - select the a attribute. Yields: {a: true}
  • a{b} - select the b sub-attribute of a (i.e. a.b). Yields: {a: {b: true}}
  • a{b,c} - select both b and c from a. Yields: {a: {b: true, c: true}}
  • a{b{c}} - select c from b from a (i.e. a.b.c). Yields: {a: {b: {c: true}}}
  • a,b{c} - select a, and the c sub-attribute of b. Yields: {a: true, b: {c: true}}.