Exposed GSON

Gson type adapters for Kotlin Exposed. For use with Exposed DAO.

Warning

This is somewhat hacky, use at your own risk (reflection and casting). It should work properly on valid DAO classes.

Issues and improvements are welcome.

Install

Exposed GSON can be included using JitPack.

Gradle

In gradle (kotlin script), it would look like this:

repositories {
    ...
    maven("https://jitpack.io")
}

dependencies {
    ...
    implementation("com.github.rnett:exposedgson:1.1.1")
}

For other build tools, see the JitPack site.

Usage

To use with a Exposed DAO class, add the annotation @JsonAdapter(ExposedTypeAdapter::class). E.x.:

@JsonAdapter(ExposedTypeAdapter::class)
class account(id: EntityID<Int>) : IntEntity(id){

Then, if you use Gson().toJson() (or any other serialization methods) on an instance of your entity, it will be serialized using Exposed GSON's type adapter.

The JSON will only include declared properties, along with a database id.

Note that it will also include properties that wouldn't normally show up in GSON JSON data.

By default, this id is the id property of the Entity superclass, and is stored in the JSON field $$database_id$$. However, these are both changeable; see @ExposedGSON.UseAsID and @ExposedGSON.JsonDatabaseIdField.

When parsing JSON, Exposed GSON only looks at the database id, then pulls the entity from the database.

Changes made in the JSON will not be reflected in the parsed object or in the database (this is something I may add in the future).

Note that seraliziation and parsing must be done inside Exposed transactions.

Annotations

Exposed GSON provides a number of annotations to customize the JSON representation of the entity. These are @ExposedGSON.JsonName, @ExposedGSON.Ignore, @ExposedGSON.UseAsID, and @ExposedGSON.JsonDatabaseIdField.

@ExposedGSON.JsonName

@ExposedGSON.JsonName can be applied to properties. It takes a string as a parameter, and uses that string as the property name in the JSON. E.x.:

@ExposedGSON.JsonName("this_email")
var email by accounts.email

email will then be represented by the JSON field this_email.

@ExposedGSON.Ignore

@ExposedGSON.Ignore can be applied to properties. It is simple: if it is present, the property will not be included in the JSON. E.x.:

@ExposedGSON.Ignore
var pwHash by accounts.pwHash

pwHash will then not be included in the JSON.

@ExposedGSON.UseAsID

@ExposedGSON.UseAsID can be applied to properties (only one per class). It tells Exposed GSON to use that property as the entity id when creating the object.

When parsing JSON, Exposed GSON will pass the value of this property to the entity class's findById method to generate the entity from the database. Make sure this is the intended behavior.

When this annotation is present, Exposed GSON will not create a separate database id field in the JSON.

If this annotation is present more than once in a class, an exception will be thrown.

This annotation will work with @ExposedGSON.JsonName.

E.x.:

object accounts : IntIdTable("accounts", "accountid") {

    // Database Columns

    val accountid = integer("accountid").autoIncrement().primaryKey()

    ...
}

@JsonAdapter(ExposedTypeAdapter::class)
class account(id: EntityID<Int>) : IntEntity(id) {
    @ExposedGSON.UseAsID
    val accountid by accounts.accountid

    ...
}

accountid will then be used in place of the special id field in the JSON.

Note that accountid is the primary key of the table, and is the correct type for findById

@ExposedGSON.JsonDatabaseIdField

@ExposedGSON.JsonDatabaseIdField can be applied to the DAO class. It is meaningless unless @JsonAdapter(ExposedTypeAdapter::class) is also used.

@ExposedGSON.JsonDatabaseIdField takes a string, and uses that string as the name of the special id field in the JSON (instead of $$database_id$$).

If both @ExposedGSON.JsonDatabaseIdField and @ExposedGSON.UseAsID are present, an exception will be thrown.

E.x.:

@JsonAdapter(ExposedTypeAdapter::class)
@ExposedGSON.JsonDatabaseIdField("this_id")
class account(id: EntityID<Int>) : IntEntity(id) {
    ...
}

Then this_id will be used as the id field in the JSON, instead of $$database_id$$.