Hyperledger Composer中关联支票簿资产及验证其存在的方法

Hey there! Let's break down your questions about Hyperledger Composer's relationship mechanism and how to validate a cheque book exists when creating a cheque.

First: Tweak Your Model Definitions for Clarity #

First, let's adjust your model files to follow common Composer naming conventions (capitalized asset/transaction names) which will make your code easier to work with:

Cheque Book Model (chequeBooks.cto) #

namespace org.example.bank.chequeBooks
asset ChequeBook identified by chequeBookNumber {
    o Integer chequeBookNumber
}

Cheque Model (cheque.cto) #

namespace org.example.bank.cheque
import org.example.bank.chequeBooks.ChequeBook

asset Cheque identified by chequeNumber {
    o Integer chequeNumber
    --> ChequeBook cb  // This creates a relationship to the ChequeBook asset
}

transaction chequeInProcess {
    o Integer chequeNumber
    o Integer chequeBookNumber  // We'll use this to look up the ChequeBook
}

event chequeCashed {
    o Integer chequeNumber
    o Integer chequeBookNumber
}

The --> ChequeBook cb line defines a relationship—this is just a reference to a ChequeBook asset (storing its unique ID, chequeBookNumber), not a copy of the entire asset. This keeps your data consistent and avoids duplication.

Second: Implement Transaction Logic to Validate & Create the Cheque #

Now, let's write the transaction processor code that will:

1. Look up the ChequeBook using the provided chequeBookNumber
2. Validate that the cheque book actually exists
3. Create a new Cheque asset linked to the valid cheque book
4. Emit the chequeCashed event

Save this as logic.js:

/**
 * Process a cheque creation transaction
 * @param {org.example.bank.cheque.chequeInProcess} tx
 * @transaction
 */
async function chequeInProcess(tx) {
    // 1. Get the asset registry for ChequeBook assets
    const chequeBookRegistry = await getAssetRegistry('org.example.bank.chequeBooks.ChequeBook');
    
    // 2. Try to fetch the ChequeBook using the provided number
    // Note: We convert the number to a string because Composer uses string IDs internally
    const chequeBook = await chequeBookRegistry.get(tx.chequeBookNumber.toString());
    
    // 3. Throw an error if the ChequeBook doesn't exist
    if (!chequeBook) {
        throw new Error(`Oops! Cheque book with number ${tx.chequeBookNumber} doesn't exist.`);
    }
    
    // 4. Create a new Cheque asset
    const factory = getFactory();
    const newCheque = factory.newResource('org.example.bank.cheque', 'Cheque', tx.chequeNumber.toString());
    
    // 5. Link the new Cheque to the existing ChequeBook using a relationship
    newCheque.cb = factory.newRelationship(
        'org.example.bank.chequeBooks', 
        'ChequeBook', 
        tx.chequeBookNumber.toString()
    );
    
    // 6. Add the new Cheque to its asset registry
    const chequeRegistry = await getAssetRegistry('org.example.bank.cheque.Cheque');
    await chequeRegistry.add(newCheque);
    
    // 7. Emit the chequeCashed event to notify external systems
    const event = factory.newEvent('org.example.bank.cheque', 'chequeCashed');
    event.chequeNumber = tx.chequeNumber;
    event.chequeBookNumber = tx.chequeBookNumber;
    emit(event);
}

Key Explanations: #

• Validating the ChequeBook: The chequeBookRegistry.get() method will throw an error if the asset doesn't exist, but we add an explicit check to throw a more user-friendly error message.
• Relationships: When creating newCheque.cb, we use factory.newRelationship() to create a reference to the existing ChequeBook—this ensures the Cheque is always linked to a valid, existing cheque book.
• String IDs: Even though your identifiers are integers, Composer stores all asset IDs as strings, so we convert the numbers to strings when fetching or creating assets.

Quick Recap of Composer Relationships #

Relationships in Composer are lightweight references between assets/participants. They:

• Store only the unique ID of the target asset (not the full asset data)
• Allow you to easily fetch the linked asset later (using the relationship)
• Ensure data consistency (you can't link to an asset that doesn't exist, as we validate in the transaction)

内容的提问来源于stack exchange，提问作者Saeedi