Insert an active model and get back the last insert id doesn't work with autoincrement

Description

Insert an active model and get back the last insert id doesn't work with autoincrement id.

Steps to Reproduce

1. Create migration and generate a model

#[derive(Clone, Debug, PartialEq, DeriveEntityModel, Eq)]
#[sea_orm(table_name = "fruit")]
pub struct Model {
    #[sea_orm(primary_key)]
    pub id: i32,
}

2. Insert an active model and get back the last insert id using static method

let pear = fruit::ActiveModel {
    ..Default::default() // just only autoincremented id here
};

let res: InsertResult = Fruit::insert(pear).exec(db).await?;

Expected Behavior

It must insert and return saved id

Actual Behavior

It leads an error:

"Custom Error: Custom Error: Attribute id is NotSet"

Reproduces How Often

All the time

Workarounds

Don't use static method and use such one instead:

let pear = fruit::ActiveModel {
    ..Default::default()
};

let pear: fruit::Model = pear.insert(db).await?;

Reproducible Example

1. Run migration:

use sea_orm_migration::prelude::*;

#[derive(DeriveMigrationName)]
pub struct Migration;

#[async_trait::async_trait]
impl MigrationTrait for Migration {
    async fn up(&self, manager: &SchemaManager) -> Result<(), DbErr> {
        manager
            .create_table(
                Table::create()
                    .table(Fruit::Table)
                    .if_not_exists()
                    .col(
                        ColumnDef::new(Fruit::Id)
                            .integer()
                            .not_null()
                            .auto_increment()
                            .primary_key(),
                    )
                    .to_owned(),
            )
            .await
    }

    async fn down(&self, manager: &SchemaManager) -> Result<(), DbErr> {
        manager
            .drop_table(Table::drop().table(Fruit::Table).to_owned())
            .await
    }
}

#[derive(DeriveIden)]
enum Fruit {
    Table,
    Id,
}

3. Generate a model based on migration above

4. Then try to insert entity using generated model:

let pear = fruit::ActiveModel {
    ..Default::default() // just only autoincremented id here
};

let res: InsertResult = Fruit::insert(pear).exec(db).await?;

Versions

0.12.14

Start at the static Fruit::insert(pear).exec(db) entry point and compare it with the working ActiveModel::insert(db) path. Trace how an autoincrement-only ActiveModel is handled and add a regression test for the reproduced migration and insert. Done means the static insert succeeds and returns the saved id instead of reporting Attribute id is NotSet.