All expressions need to have this getter for complicated type-related reasons.
Simply add this getter for your expression and always return undefined from it:
import { type Expression, type OperationNode, sql } from 'kysely'
class SomeExpression<T> implements Expression<T> {
get expressionType(): T | undefined {
return undefined
}
toOperationNode(): OperationNode {
return sql`some sql here`.toOperationNode()
}
}
The getter is needed to make the expression assignable to another expression only
if the types T are assignable. Without this property (or some other property
that references T), you could assing Expression<string> to Expression<number>.
Returns an aliased version of the expression.
In addition to slapping as "the_alias" at the end of the expression,
this method also provides strict typing:
const result = await db
.selectFrom('person')
.select((eb) =>
// `eb.fn<string>` returns an AliasableExpression<string>
eb.fn<string>('concat', ['first_name', eb.val(' '), 'last_name']).as('full_name')
)
.executeTakeFirstOrThrow()
// `full_name: string` field exists in the result type.
console.log(result.full_name)
The generated SQL (PostgreSQL):
select
concat("first_name", $1, "last_name") as "full_name"
from
"person"
You can also pass in a raw SQL snippet (or any expression) but in that case you must provide the alias as the only type argument:
import { sql } from 'kysely'
const values = sql<{ a: number, b: string }>`(values (1, 'foo'))`
// The alias is `t(a, b)` which specifies the column names
// in addition to the table name. We must tell kysely that
// columns of the table can be referenced through `t`
// by providing an explicit type argument.
const aliasedValues = values.as<'t'>(sql`t(a, b)`)
await db
.insertInto('person')
.columns(['first_name', 'last_name'])
.expression(
db.selectFrom(aliasedValues).select(['t.a', 't.b'])
)
.execute()
The generated SQL (PostgreSQL):
insert into "person" ("first_name", "last_name")
from (values (1, 'foo')) as t(a, b)
select "t"."a", "t"."b"
Creates the OperationNode that describes how to compile this expression into SQL.
If you are creating a custom expression, it's often easiest to use the sql template tag to build the node:
import { type Expression, type OperationNode, sql } from 'kysely'
class SomeExpression<T> implements Expression<T> {
get expressionType(): T | undefined {
return undefined
}
toOperationNode(): OperationNode {
return sql`some sql here`.toOperationNode()
}
}
An expression with an
asmethod.