Computed columns

Discussed in https://github.com/SeaQL/sea-orm/discussions/1803

^{Originally posted by mara-schulke January 29, 2023}

Motivation

Ive integrated sea orm in a quite complex project where the need arises for computed columns from different tables.

Proposed Solutions

A potential solution could be to have a computed column annotated on the model (which is not included in the derived active model) and just link a function which gets called on every selection of the model:

#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize, DeriveEntityModel)]
#[sea_orm(table_name = "invoice_line_item")]
pub struct Model {
    #[sea_orm(primary_key)]
    pub id: i64,

    #[sea_orm(foreign_key)]
    pub invoice_id: i64,
     
    // ..  
  
    pub amount: i64,

    pub created_at: DateTime<Utc>,
    pub updated_at: DateTime<Utc>,
}

#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize, DeriveEntityModel)]
#[sea_orm(table_name = "invoice")]
pub struct Model {
    #[sea_orm(primary_key)]
    pub id: i64,

    #[sea_orm(compute = "Entity::total")]
    pub total: i64,

    pub created_at: DateTime<Utc>,
    pub updated_at: DateTime<Utc>,
}

impl Entity {
    async fn total(pk: i64) -> Result<i64, sea_orm::error::DbErr> {
        // Run query which sums all related invoice line items
    }
}

Additional Information

I think this would be pretty useful to a lot of realworld applications, what do you think about this? 🙂

Start with Discussion #1803 and the DeriveEntityModel examples included in the issue; no repository file or test is identified. The scope and completion criteria need maintainer agreement on how computed fields should be represented and queried.