Agent skill
drizzle-orm
Drizzle ORM patterns for type branding and custom types. Use when working with Drizzle column definitions, branded types, or custom type conversions.
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SKILL.md
Drizzle ORM Guidelines
Use $type<T>() for Branded Strings, Not customType
When you need a column with a branded TypeScript type but no actual data transformation, use $type<T>() instead of customType.
The Rule
If toDriver and fromDriver would be identity functions (x) => x, use $type<T>() instead.
Why
Even with identity functions, customType still invokes mapFromDriverValue on every row:
// drizzle-orm/src/utils.ts - runs for EVERY column of EVERY row
const rawValue = row[columnIndex]!;
const value = rawValue === null ? null : decoder.mapFromDriverValue(rawValue);
Query 1000 rows with 3 date columns = 3000 function calls doing nothing.
Bad Pattern
// Runtime overhead for identity functions
customType<{ data: DateTimeString; driverParam: DateTimeString }>({
dataType: () => 'text',
toDriver: (value) => value, // called on every write
fromDriver: (value) => value, // called on every read
});
Good Pattern
// Zero runtime overhead - pure type assertion
text().$type<DateTimeString>();
$type<T>() is a compile-time-only type override:
// drizzle-orm/src/column-builder.ts
$type<TType>(): $Type<this, TType> {
return this as $Type<this, TType>;
}
When to Use customType
Only when data genuinely transforms between app and database:
// JSON: object ↔ string - actual transformation
customType<{ data: UserPrefs; driverParam: string }>({
toDriver: (value) => JSON.stringify(value),
fromDriver: (value) => JSON.parse(value),
});
Keep Data in Intermediate Representation
Prefer keeping data serialized (strings) through the system, parsing only at the edges (UI components).
The principle: If data enters serialized and leaves serialized, keep it serialized in the middle. Parse at the edges where you actually need the rich representation.
Example: DateTimeString
Instead of parsing DateTimeString into Temporal.ZonedDateTime at the database layer:
// Bad: parse on every read, re-serialize at API boundaries
customType<{ data: Temporal.ZonedDateTime; driverParam: string }>({
fromDriver: (value) => fromDateTimeString(value),
});
Keep it as a string until the UI actually needs it:
// Good: string stays string, parse only in date-picker component
text().$type<DateTimeString>();
// In UI component:
const temporal = fromDateTimeString(row.createdAt);
// After edit:
const updated = toDateTimeString(temporal);
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