Foreign Key Validation

QAIL provides compile-time validation for foreign key references, ensuring your schema is consistent before migrations run.

Defining Foreign Keys

Use the builder API to define foreign key constraints:

#![allow(unused)]
fn main() {
use qail_core::migrate::{Column, ColumnType, ForeignKey, FkAction};

let user_id = Column::new("user_id", ColumnType::Uuid)
    .references("users", "id")
    .on_delete(FkAction::Cascade)
    .on_update(FkAction::NoAction);
}

FK Actions

Action	SQL	Description
`FkAction::NoAction`	`NO ACTION`	Reject if referenced row exists (default)
`FkAction::Cascade`	`CASCADE`	Delete/update child rows
`FkAction::SetNull`	`SET NULL`	Set FK column to NULL
`FkAction::SetDefault`	`SET DEFAULT`	Set FK column to default value
`FkAction::Restrict`	`RESTRICT`	Same as NO ACTION but checked immediately

Schema Validation

Call validate() to check all FK references exist:

#![allow(unused)]
fn main() {
let mut schema = Schema::new();

schema.add_table(Table::new("users")
    .column(Column::new("id", ColumnType::Uuid).primary_key()));

schema.add_table(Table::new("posts")
    .column(Column::new("id", ColumnType::Uuid).primary_key())
    .column(Column::new("user_id", ColumnType::Uuid)
        .references("users", "id")));

// Validate all FK references
match schema.validate() {
    Ok(()) => println!("Schema is valid"),
    Err(errors) => {
        for e in errors {
            eprintln!("Error: {}", e);
        }
    }
}
}

Error Messages

If a FK references a non-existent table or column:

FK error: posts.user_id references non-existent table 'users'
FK error: posts.author_id references non-existent column 'users.author_id'

Best Practices

Always validate before migrating

#![allow(unused)]
fn main() {
let schema = parse_qail(&content)?;
schema.validate()?;
}

Use Cascade carefully - it can delete more data than expected

Prefer SetNull for optional relationships

#![allow(unused)]
fn main() {
.references("categories", "id")
.on_delete(FkAction::SetNull)
}