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Fluent's schema API allows you to create and update your database schema programatically. It is often used in conjunction with migrations to prepare the database for use with models.

// An example of Fluent's schema API
    .field("name", .string, .required)
    .field("star_id", .uuid, .required, .references("stars", "id"))

To create a SchemaBuilder, use the schema method on database. Pass in the name of the table or collection you want to affect. If you are editing the schema for a model, make sure this name matches the model's schema.


The schema API supports creating, updating, and deleting schemas. Each action supports a subset of the API's available methods.


Calling create() creates a new table or collection in the database. All methods for defining new fields and constraints are supported. Methods for updates or deletes are ignored.

// An example schema creation.
    .field("name", .string, .required)

If a table or collection with the chosen name already exists, an error will be thrown. To ignore this, use .ignoreExisting().


Calling update() updates an existing table or collection in the database. All methods for creating, updating, and deleting fields and constraints are supported.

// An example schema update.
    .unique(on: "name")


Calling delete() deletes an existing table or collection from the database. No additional methods are supported.

// An example schema deletion.


Fields can be added when creating or updating a schema.

// Adds a new field
.field("name", .string, .required)

The first parameter is the name of the field. This should match the key used on the associated model property. The second parameter is the field's data type. Finally, zero or more constraints can be added.

Data Type

Supported field data types are listed below.

DataType Swift Type
.string String
.int{8,16,32,64} Int{8,16,32,64}
.uint{8,16,32,64} UInt{8,16,32,64}
.bool Bool
.datetime Date (recommended)
.time Date (omitting day, month, and year)
.date Date (omitting time of day)
.float Float
.double Double
.data Data
.uuid UUID
.dictionary See dictionary
.array See array
.enum See enum

Field Constraint

Supported field constraints are listed below.

FieldConstraint Description
.required Disallows nil values.
.references Requires that this field's value match a value in the referenced schema. See foreign key
.identifier Denotes the primary key. See identifier


If your model uses a standard @ID property, you can use the id() helper to create its field. This uses the special .id field key and UUID value type.

// Adds field for default identifier.

For custom identifier types, you will need to specify the field manually.

// Adds field for custom identifier.
.field("id", .int, .identifier(auto: true))

The identifier constraint may be used on a single field and denotes the primary key. The auto flag determines whether or not the database should generate this value automatically.

Update Field

You can update a field's data type using updateField.

// Updates the field to `double` data type.
.updateField("age", .double)

See advanced for more information on advanced schema updates.

Delete Field

You can remove a field from a schema using deleteField.

// Deletes the field "age".


Constraints can be added when creating or updating a schema. Unlike field constraints, top-level constraints can affect multiple fields.


A unique constraint requires that there are no duplicate values in one or more fields.

// Disallow duplicate email addresses.
.unique(on: "email")

If multiple field are constrained, the specific combination of each field's value must be unique.

// Disallow users with the same full name.
.unique(on: "first_name", "last_name")

To delete a unique constraint, use deleteUnique.

// Removes duplicate email constraint.
.deleteUnique(on: "email")

Constraint Name

Fluent will generate unique constraint names by default. However, you may want to pass a custom constraint name. You can do this using the name parameter.

// Disallow duplicate email addresses.
.unique(on: "email", name: "no_duplicate_emails")

To delete a named constraint, you must use deleteConstraint(name:).

// Removes duplicate email constraint.
.deleteConstraint(name: "no_duplicate_emails")

Foreign Key

Foreign key constraints require that a field's value match ones of the values in the referenced field. This is useful for preventing invalid data from being saved. Foreign key constraints can be added as either a field or top-level constraint.

To add a foreign key constraint to a field, use .references.

// Example of adding a field foreign key constraint.
.field("star_id", .uuid, .required, .references("stars", "id"))

The above constraint requires that all values in the "star_id" field must match one of the values in Star's "id" field.

This same constraint could be added as a top-level constraint using foreignKey.

// Example of adding a top-level foreign key constraint.
.foreignKey("star_id", references: "star", "id")

Unlike field constraints, top-level constraints can be added in a schema update. They can also be named.

Foreign key constraints support optional onDelete and onUpdate actions.

ForeignKeyAction Description
.noAction Prevents foreign key violations (default).
.restrict Same as .noAction.
.cascade Propogates deletes through foreign keys.
.setNull Sets field to null if reference is broken.
.setDefault Sets field to default if reference is broken.

Below is an example using foreign key actions.

// Example of adding a top-level foreign key constraint.
.foreignKey("star_id", references: "star", "id", onDelete: .cascade)


The dictionary data type is capable of storing nested dictionary values. This includes structs that conform to Codable and Swift dictionaries with a Codable value.


Fluent's SQL database drivers store nested dictionaries in JSON columns.

Take the following Codable struct.

struct Pet: Codable {
    var name: String
    var age: Int

Since this Pet struct is Codable, it can be stored in a @Field.

@Field(key: "pet")
var pet: Pet

This field can be stored using the .dictionary(of:) data type.

.field("pet", .dictionary, .required)

Since Codable types are heterogenous dictionaries, we do not specify the of parameter.

If the dictionary values were homogenous, for example [String: Int], the of parameter would specify the value type.

.field("numbers", .dictionary(of: .int), .required)

Dictionary keys must always be strings.


The array data type is capable of storing nested arrays. This includes Swift arrays that contain Codable values and Codable types that use an unkeyed container.

Take the following @Field that stores an array of strings.

@Field(key: "tags")
var tags: [String]

This field can be stored using the .array(of:) data type.

.field("tags", .array(of: .string), .required)

Since the array is homogenous, we specify the of parameter.

Codable Swift Arrays will always have a homogenous value type. Custom Codable types that serialize heterogenous values to unkeyed containers are the exception and should use the .array data type.


The enum data type is capable of storing string backed Swift enums natively. Native database enums provide an added layer of type safety to your database and may be more performant than raw enums.

To define a native database enum, use the enum method on Database. Use case to define each case of the enum.

// An example of enum creation.

Once an enum has been created, you can use the read() method to generate a data type for your schema field.

// An example of reading an enum and using it to define a new field.
database.enum("planet_type").read().flatMap { planetType in
        .field("type", planetType, .required)

To update an enum, call update(). Cases can be deleted from existing enums.

// An example of enum update.

To delete an enum, call delete().

// An example of enum deletion.

Model Coupling

Schema building is purposefully decoupled from models. Unlike query building, schema building does not make use of key paths and is completely stringly typed. This is important since schema definitions, especially those written for migrations, may need to reference model properties that no longer exist.

To better understand this, take a look at the following example migration.

struct UserMigration: Migration {
    func prepare(on database: Database) -> EventLoopFuture<Void> {
            .field("id", .uuid, .identifier(auto: false))
            .field("name", .string, .required)

    func revert(on database: Database) -> EventLoopFuture<Void> {

Let's assume that this migration has been has already been pushed to production. Now let's assume we need to make the following change to the User model.

- @Field(key: "name")
- var name: String
+ @Field(key: "first_name")
+ var firstName: String
+ @Field(key: "last_name")
+ var lastName: String

We can make the necessary database schema adjustments with the following migration.

struct UserNameMigration: Migration {
    func prepare(on database: Database) -> EventLoopFuture<Void> {
            .field("first_name", .string)
            .field("last_name", .string)

    func revert(on database: Database) -> EventLoopFuture<Void> {

Note that for this migration to work, we need to be able to reference both the removed name field and the new firstName and lastName fields at the same time. Furthermore, the original UserMigration should continue to be valid. This would not be possible to do with key paths.