UUVPN/iOS-SwiftUI-Code/GRDB.swift-6.29.3/GRDB/Documentation.docc/Extension/DatabaseValueConvertible.md

# ``GRDB/DatabaseValueConvertible``

A type that can convert itself into and out of a database value.

## Overview

A `DatabaseValueConvertible` type supports conversion to and from database values (null, integers, doubles, strings, and blobs). `DatabaseValueConvertible` is adopted by `Bool`, `Int`, `String`, `Date`, etc.

> Note: Types that converts to and from multiple columns in a database row must not conform to the `DatabaseValueConvertible` protocol. Those types are called **record types**, and should conform to record protocols instead. See <doc:QueryInterface>.

> Note: Standard collections `Array`, `Set`, and `Dictionary` do not conform to `DatabaseValueConvertible`. To store arrays, sets, or dictionaries in individual database values, wrap them as properties of `Codable` record types. They will automatically be stored as JSON objects and arrays. See <doc:QueryInterface>.

## Conforming to the DatabaseValueConvertible Protocol

To conform to `DatabaseValueConvertible`, implement the two requirements ``fromDatabaseValue(_:)-21zzv`` and ``databaseValue-1ob9k``. Do not customize the ``fromMissingColumn()-7iamp`` requirement. If your type `MyValue` conforms, then the conformance of the optional type `MyValue?` is automatic.

The implementation of `fromDatabaseValue` must return nil if the type can not be decoded from the raw database value. This nil value will have GRDB throw a decoding error accordingly.

For example:

```swift
struct EvenInteger {
    let value: Int // Guaranteed even

    init?(_ value: Int) {
        guard value.isMultiple(of: 2) else {
            return nil // Not an even number
        }
        self.value = value
    }
}

extension EvenInteger: DatabaseValueConvertible {
    var databaseValue: DatabaseValue {
        value.databaseValue
    }

    static func fromDatabaseValue(_ dbValue: DatabaseValue) -> Self? {
        guard let value = Int.fromDatabaseValue(dbValue) else {
            return nil // Not an integer
        }
        return EvenInteger(value) // Nil if not even
    }
}
```

### Built-in RawRepresentable support

`DatabaseValueConvertible` implementation is ready-made for `RawRepresentable` types whose raw value is itself `DatabaseValueConvertible`, such as enums:

```swift
enum Grape: String {
    case chardonnay, merlot, riesling
}

// Encodes and decodes `Grape` as a string in the database:
extension Grape: DatabaseValueConvertible { }
```

### Built-in Codable support

`DatabaseValueConvertible` is also ready-made for `Codable` types, which are automatically coded and decoded from JSON arrays and objects:

```swift
struct Color: Codable {
    var red: Double
    var green: Double
    var blue: Double
}

// Encodes and decodes `Color` as a JSON object in the database:
extension Color: DatabaseValueConvertible { }
```

By default, such codable value types are encoded and decoded with the standard [JSONEncoder](https://developer.apple.com/documentation/foundation/jsonencoder) and [JSONDecoder](https://developer.apple.com/documentation/foundation/jsondecoder). `Data` values are handled with the `.base64` strategy, `Date` with the `.millisecondsSince1970` strategy, and non conforming floats with the `.throw` strategy.

To customize the JSON format, provide an explicit implementation for the `DatabaseValueConvertible` requirements, or implement these two methods:

```swift
protocol DatabaseValueConvertible {
    static func databaseJSONDecoder() -> JSONDecoder
    static func databaseJSONEncoder() -> JSONEncoder
}
```

### Adding support for the Tagged library

[Tagged](https://github.com/pointfreeco/swift-tagged) is a popular library that makes it possible to enhance the type-safety of our programs with dedicated wrappers around basic types. For example:

```swift
import Tagged

struct Player: Identifiable {
    // Thanks to Tagged, Player.ID can not be mismatched with Team.ID or
    // Award.ID, even though they all wrap strings.
    typealias ID = Tagged<Player, String>
    var id: ID
    var name: String
    var score: Int
}
```

Applications that use both Tagged and GRDB will want to add those lines somewhere:

```swift
import GRDB
import Tagged

// Add database support to Tagged values
extension Tagged: SQLExpressible where RawValue: SQLExpressible { }
extension Tagged: StatementBinding where RawValue: StatementBinding { }
extension Tagged: StatementColumnConvertible where RawValue: StatementColumnConvertible { }
extension Tagged: DatabaseValueConvertible where RawValue: DatabaseValueConvertible { }
```

This makes it possible to use `Tagged` values in all the expected places:

```swift
let id: Player.ID = ...
let player = try Player.find(db, id: id)
```

## Optimized Values

For extra performance, custom value types can conform to both `DatabaseValueConvertible` and ``StatementColumnConvertible``. This extra protocol grants raw access to the [low-level C SQLite interface](https://www.sqlite.org/c3ref/column_blob.html) when decoding values.

For example:

```swift
extension EvenInteger: StatementColumnConvertible {
    init?(sqliteStatement: SQLiteStatement, index: CInt) {
        let int64 = sqlite3_column_int64(sqliteStatement, index)
        guard let value = Int(exactly: int64) else {
            return nil // Does not fit Int (probably a 32-bit architecture)
        }
        self.init(value) // Nil if not even
    }
}
```

This extra conformance is not required: only aim at the low-level C interface if you have identified a performance issue after profiling your application! 

## Topics

### Creating a Value

- ``fromDatabaseValue(_:)-21zzv``
- ``fromMissingColumn()-7iamp``

### Accessing the DatabaseValue

- ``databaseValue-1ob9k``

### Configuring the JSON format for the standard Decodable protocol

- ``databaseJSONDecoder()-7zou9``
- ``databaseJSONEncoder()-37sff``

### Fetching Values from Raw SQL

- ``fetchCursor(_:sql:arguments:adapter:)-6elcz``
- ``fetchAll(_:sql:arguments:adapter:)-1cqyb``
- ``fetchSet(_:sql:arguments:adapter:)-5jene``
- ``fetchOne(_:sql:arguments:adapter:)-qvqp``

### Fetching Values from a Prepared Statement

- ``fetchCursor(_:arguments:adapter:)-4l6af``
- ``fetchAll(_:arguments:adapter:)-3abuc``
- ``fetchSet(_:arguments:adapter:)-6y54n``
- ``fetchOne(_:arguments:adapter:)-3d7ax``

### Fetching Values from a Request

- ``fetchCursor(_:_:)-8q4r6``
- ``fetchAll(_:_:)-9hkqs``
- ``fetchSet(_:_:)-1foke``
- ``fetchOne(_:_:)-o6yj``

### Supporting Types

- ``DatabaseValueCursor``
- ``StatementBinding``