Enum Change 

pub enum Change<T> {
    Leave,
    Set(T),
}

Whether a partial update touches a field.

The third state a plain Option cannot carry: for a nullable column, “leave it alone” and “set it to NULL” are different requests, and Option<T> has one spelling for both.

Change::Leave leaves the column out of the SET clause, and Change::Set writes the inner value — including null. They are separate constructors, so there is no third thing to write: no way to pass a value that is missing rather than null.

Clearability lives in the inner type rather than in a third variant, so it mirrors the column: Change<Option<i32>> can leave, clear or set, while Change<bool> can only leave or set, because the column behind it is NOT NULL.

Variants

Leave

Leave whatever is stored.

Set(T)

Store this instead.

Implementations

impl<T> Change<T>

pub fn set(self) -> Option<T>

The value to store, if this changes anything.

pub fn is_leave(&self) -> bool

Whether this leaves the field alone.

Trait Implementations

impl<T: Clone> Clone for Change<T>

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T: Copy> Copy for Change<T>

impl<T: Debug> Debug for Change<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T> Default for Change<T>

fn default() -> Self

Leaves the field alone, so ..Default::default() narrows an update rather than widening it. Hand-written because deriving would demand T: Default, which says nothing here.

impl<T: Eq> Eq for Change<T>

impl<T: PartialEq> PartialEq for Change<T>

fn eq(&self, other: &Self) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl<T: PartialEq> StructuralPartialEq for Change<T>