Constraints

Constraints modify field behavior. They attach to field types in a model block.

required

Column is NOT NULL. Empty form submissions are rejected with a validation error.

name: text required

unique

Column has a UNIQUE index. Duplicate INSERT/UPDATE is rejected.

email: email unique
slug: text required unique

Typical pairing: required unique for identity fields like username or slug.

default <value>

Inserts the given value when the field is unset.

role: option [admin, editor, viewer] default viewer
active: bool default true
views: int default 0

Quoted strings are supported for text defaults:

status: text default "pending"

auto

Auto-generates a value on INSERT. Behavior depends on field type:

Field type	Behavior
`timestamp`	Current UTC time
`date`	Today's date
`uuid`	UUID v4
`bool`	`false`

model order
  created: timestamp auto
  placed_on: date auto
  reference: uuid auto

auto_update

Emits a database trigger that auto-updates the column on every UPDATE statement. Used for updated_at patterns.

model post
  title: text required
  updated: timestamp auto_update

SQLite generates an AFTER UPDATE trigger. PostgreSQL generates a BEFORE UPDATE trigger with a plpgsql function. The trigger is created/replaced automatically during migration. Applies to timestamp fields; other types will have the trigger defined but may not produce meaningful values.

auto_update differs from auto — auto runs only on INSERT, auto_update runs on every UPDATE.

min <n>

Minimum length (for text types) or minimum value (for numeric types).

name: text required min 2
age: int min 18

Form validation rejects values below the minimum. For text, measured in characters.

max <n>

Maximum length or value.

title: text required max 200
score: int max 100

Composite unique

For uniqueness across two or more fields, declare a model-level unique (...) directive instead of marking a single field unique:

model membership
  user: user required
  project: project required
  role: option [owner, admin, member] default member
  unique (user, project)

Rules:

At least two fields. Single-column uniqueness uses the field-level unique constraint.
Fields must be declared on the same model. Reference fields resolve to their <name>_id column automatically.
Multiple unique (...) lines are allowed for independent composite groups.

Migration emits CREATE UNIQUE INDEX IF NOT EXISTS "uq_<table>_<col>_<col>" ON "<table>" (...), safe to rerun on both SQLite and PostgreSQL.

Non-unique indexes

For query acceleration without a uniqueness requirement, declare an index (...) directive inside the model. Single-column and multi-column are both valid:

model order
  customer: customer required
  created: timestamp auto
  status: option [pending, paid, shipped]
  index (customer, created)
  index (status)

Migration emits CREATE INDEX IF NOT EXISTS "ix_<table>_<col>_<col>" ON "<table>" (...). The ix_ prefix keeps non-unique indexes distinguishable from composite UNIQUE indexes (uq_) in database tooling and migration history. kilnx check rejects unknown field names, fields repeated within a group, and duplicated groups.

Combining constraints

Constraints appear space-separated after the field type:

model user
  email: email required unique
  age: int required min 13 max 120
  bio: text max 500 default ""
  role: option [admin, user] default user required
  created: timestamp auto
  updated: timestamp auto_update

Order is not significant.

Summary

Constraint	Applies to	Effect
`required`	All	NOT NULL
`unique`	All	UNIQUE index
`default <val>`	All	Default value
`auto`	`timestamp`, `date`, `uuid`, `bool`	Auto-generate on INSERT
`auto_update`	`timestamp`	DB trigger: update on every UPDATE
`min <n>`	Text (length) or numeric (value)	Lower bound
`max <n>`	Text (length) or numeric (value)	Upper bound
`unique (a, b, ...)`	Model-level, 2+ fields	Composite UNIQUE index
`index (a, b, ...)`	Model-level, 1+ fields	Non-unique index

Total: 7 field constraints plus 2 model-level directives.