Handling optional fields¶
It is possible to patch using optional values of type Option[T]
as long as the transformer is available for T. If the value
is present (Some), it’s used for patching a field in the target object;
otherwise (None) it’s ignored and the field value is copied from
the original object.
Let’s consider the following patch.
case class User(id: Int, email: String, phone: Long)
case class UserPatch(email: Option[String], phone: Option[Long])
Then it is possible to patch as follows:
val user = User(10, "abc@@domain.com", 1234567890L)
val update = UserPatch(email = Some("updated@@example.com"), phone = None)
user.patchUsing(update)
// User(10, "updated@@example.com", 1234567890L)
Field phone remained the same as in the original user, while
the optional e-mail string got updated from a patch object.
Option[T] on both sides¶
An interesting case appears when both patch case class and patched
object define fields f: Option[T]. Depending on values
of f in patched object and patch, we would like to
apply following semantic table.
|
|
patching result |
|---|---|---|
|
|
|
|
|
|
|
|
|
|
|
??? |
When a patch.f contains some value, it’s immediately used for
replacing field in target object (rows 1 and 2), regardless of original
object field value. When both field are None, patching result
is also None (row 3).
But if original object contains a some value, but patch comes
with a None, we can do two things:
clear value in target object (replace it with
None)or ignore updating this particular field (as in previous section)
Both choices may have perfect sense, depending on the context.
By default, Chimney does the former (clears the value), but it
also gives a simple way to always ignore None from patch
with .ignoreNoneInPatch operation.
case class User(name: Option[String], age: Option[Int])
case class UserPatch(name: Option[String], age: Option[Int])
val user = User(Some("John"), Some(30))
val userPatch = UserPatch(None, None)
user.patchUsing(userPatch)
// clears both fields: User(None, None)
user
.using(userPatch)
.ignoreNoneInPatch
.patch
// ignores updating both fields: User(Some("John"), Some(30))