Ash has three concepts that map to the concept of a virtual field. Often, virtual fields exist to simulate one of these three things
Arguments
Arguments are action-specific inputs. For example:
create :register do
accept [:email]
argument :password, :string, allow_nil?: false
argument :confirm_password, :string, allow_nil?: false
validate confirm(:password, :confirm_password)
change EncryptPassword
end
In the example above there is no “password” attribute, but we have two arguments that we use to ultimately write to that attribute.
Calculations
Another thing virtual fields are used for in ecto is storing computed values. In Ash we use calculations for that. For example:
calculations do
calculate :full_name, :string, expr(first_name <> " " <> last_name)
end
Action-specific metadata
Finally, we have action-specific metadata. An example might be returning an authentication token from a sign in action (something ash_authentication does)
read :sign_in do
get? true
# this just declares that some metadata will be placed on the result called `token`
metadata :token, :string, allow_nil?: false
... logic for signing in.
prepare CreateAndSetToken # some logic to create and set that token metadata
end
Then the result of that will be in record.__metadata__.token.
Hope that helps!
