# The Asset Model

To avoid redundant data in transactions, the asset model is different for `CREATE` and `TRANSFER` transactions.

## In CREATE Transactions

In a `CREATE` transaction, the `"asset"` must contain exactly one key-value pair. The key must be `"data"` and the value can be any valid JSON document, or `null`. For example:
```json
{
    "data": {
                "desc": "Gold-inlay bookmark owned by Xavier Bellomat Dickens III",
                "xbd_collection_id": 1857
            }
}
```

When using MongoDB for storage, certain restriction apply to all (including nested) keys of the `"data"` JSON document:

* Keys (i.e. key names, not values) must **not** begin with the `$` character.
* Keys must not contain `.` or the null character (Unicode code point 0000).
* The key `"language"` (at any level in the hierarchy) is a special key and used for specifying text search language. Its value must be one of the allowed values; see the valid [Text Search Languages](https://docs.mongodb.com/manual/reference/text-search-languages/) in the MongoDB Docs. In BigchainDB, only the languages supported by _MongoDB community edition_ are allowed.


## In TRANSFER Transactions

In a `TRANSFER` transaction, the `"asset"` must contain exactly one key-value pair. They key must be `"id"` and the value must contain a transaction ID (i.e. a SHA3-256 hash: the ID of the `CREATE` transaction which created the asset, which also serves as the asset ID). For example:
```json
{
    "id": "38100137cea87fb9bd751e2372abb2c73e7d5bcf39d940a5516a324d9c7fb88d"
}
```