# Serializer / Deserializer

### Serializer

You can resolve value of dynamoProperty using by sourceObject.  For example, `alias : id` is resolved to `[x, y].join("_")` It is good for generate `composite key`.

```typescript
@DynamoEntity()
class Entity {
    @DynamoProperty({
        keyType: KeyType.hash,
        propertyName: "id",
        serializer: (arg: SerializerArg<Entity>) => {  // <<<
            const source: Entity = arg.source;
            return [source.x, source.y].join("_");
        }
    })
    __id?: string;

    x: string;
    y: string;

    constructor(x: string, y: string) {
        this.x = x;
        this.y = y;
    }
}
const entity = new Entity("Hello", "World!");
const dynamo = Mapper.formation(entity, Entity);
```

It will be formationed as,

```typescript
{
    "id": {
        "S": "Hello_World!"
    }
}
```

### Deserailizer

Also you can revert using by value of DynamoItem. The `deserializer` returns fragment to return to the original object. You can have only one Serializer and only one Deserializer in each Property. All fragments are merged.

```typescript
@DynamoEntity()
class Entity {
    @DynamoProperty({
        keyType: KeyType.hash,
        propertyName: "id",
        serializer: (arg: SerializerArg<Entity>) => {
            const source: Entity = arg.source;
            return [source.x, source.y].join("_");
        },
        deserializer: (arg: DeserializerArg): Partial<Entity> => { // <<<
          const token: string[] = arg.dynamo.id.S!!.split("_");
          return {
              x: token[0],
              y: token[1]
          };
      }
    })
    __id?: string;

    x: string;
    y: string;

    constructor(x: string, y: string) {
        this.x = x;
        this.y = y;
    }
}
```


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