ActiveExprModel: insert and update a model with SimpleExpr

[billy1624]

Motivation

It'd be convenient if we allow inserting and updating with expression such as summation of two columns "num1" + "num2" or even DB built in function CURRENT_TIMESTAMP or UUID().

Proposed Solutions

Introduce a SimpleExpr version of ActiveValue and ActiveModel. Which conversion methods will be provided to convert ActiveValue into ActiveExpr and ActiveModel into ActiveExprModel.

let active_expr_model: ActiveExprModel = 
    ActiveModel {
        // ...
    }
    .into_active_expr_model();

let model: Model =
    ActiveExprModel {
        created_at: SetExpr( ... ),
    }
    .insert(db) // or `.update(db)`
    .await?;

[billy1624]

Comments? @tyt2y3

[tyt2y3]

I second the idea of having an ergonimc way to SET c = c+1.

However I think ActiveExprModel is not necessary.

The API can probably look like:

let change_set = ChangeSet::new(Cake).set(cake::Column::Counter, cake::Column::Counter.add(1));
change_set.insert(&db).await?; // lets pretend the expression is a function call

change_set
    .which(2) // this is the PK, basically where PK = val
    .where(SimpleExpr)
    .update(&db).await?; // if `which` or `where` is never called, we should refuse to execute

[tgrushka]

What about something like ... ?

ActiveValue::SetExpr(Expr::current_timestamp())