11 / 15Module 3: Create the marketplaceAdvanced

Charge a fee for mystery card purchases

The minting lesson introduced operation buy_mystery_card, which mints a random card for the caller. To turn it into a real purchase, you need to define a price and charge the user that amount each time the operation runs.

Each payment moves from the user to a dedicated dapp account that collects all card-sale revenue.

That economy account does not exist yet, so you will create it with FT4. Add a new entity, dapp_account, to hold it. This lets your Rell code reach the account directly.

First, define a structure in Rell that references the account.

src/rell_marketplace/module.rell

struct module_args {    dapp_account_signer: pubkey;}

Next, create the account inside the dapp_meta object you declared earlier.

src/rell_marketplace/module.rell

object dapp_meta {    asset = Unsafe.register_asset("Collector Card", "CRD", 6, chain_context.blockchain_rid, "https://url-to-asset-icon");    account = create_account_with_auth(single_sig_auth_descriptor(chain_context.args.dapp_account_signer, set(["A", "T"])));}

This code calls create_account_with_auth to create the account and passes it a single-signature auth descriptor with the following values:

The dapp account pubkey, read from the moduleArgs settings
The flags "A", which allows managing accounts, and "T", which allows token transfers from this account

Now generate a new keypair to act as the dapp's "account owner":

shell

chr keygen --file .chromia/dapp-account

Open chromia.yaml, add a module argument for the economy account, and set its value to the pubkey.

chromia.yml

blockchains:  rell_marketplace:    module: main    moduleArgs:      lib.ft4.core.accounts:        rate_limit:          max_points: 10          recovery_time: 5000          points_at_account_creation: 2        lib.ft4.core.admin:          admin_pubkey: x"<dapp_account_pubkey>" # Replace with previously generated public key here.        rell_marketplace:          dapp_account_signer: x"<dapp_account_pubkey>" # Replace with previously generated public key here.compile:  rellVersion: 0.14.9database:  schema: schema_rell_marketplacetest:  modules:    - rell_marketplace_test  moduleArgs:    rell_marketplace:      dapp_account_signer: x"<dapp_account_pubkey>" # Replace with previously generated public key here.    lib.ft4.core.admin:        admin_pubkey: x"<dapp_account_pubkey>" # Replace with previously generated public key here.libs:  ft4:    registry: https://gitlab.com/chromaway/ft4-lib.git    path: rell/src/lib/ft4    tagOrBranch: v1.2.0r    rid: x"E96915FD7C9A5DD4E183D86F4A5B4BB5083A7CC1EDEB1465EDFFC926E67C93F6"    insecure: false  iccf:    registry: https://gitlab.com/chromaway/core/directory-chain    path: src/lib/iccf    tagOrBranch: 1.87.0    rid: x"9C359787B75927733034EA1CEE74EEC8829D2907E4FC94790B5E9ABE4396575D"    insecure: false

You now have an account that can receive charges. When a user buys a card, for example, the amount moves from their account to this one.

To apply this, add a transfer to the mystery card purchase. The transfer takes these parameters:

From account
To account
Asset
Amount

With the transfer in place, buy_mystery_card looks like this:

src/rell_marketplace/marketplace.rell

operation buy_mystery_card() {    val account = auth.authenticate();    Unsafe.transfer(account, dapp_meta.account, dapp_meta.asset, 100);    mint_card(account);}

The card price is fixed at 100 CRD. Each call moves 100 CRD from the authenticated account to your dapp_account.

Written for this site from Chromia’s public course. Code samples are the course’s own.