Modify data

This chapter provides a detailed explanation and step-by-step guide for modifying data within an indexed composition in Inery.

Before Proceeding

Ensure that you are familiar with the indexed composition definitions and insertion methods covered in the Insert Data section.

Data Modification Process

Modifying data in a composition involves verifying if the target object exists within the indexed composition. If it exists, you can modify it using the modify method. Below are the key steps:

• Verify if the Object Exists: Use an iterator to check for the presence of the object.
• Modify the Object: Update the object using the modify method if it is found in the indexed composition.

Find Record to Modify

Define logic for locating the record in the indexed composition. Add custom error handling if the record does not exist.

Full Modify Action

Full Modify Action

ACTION modify(name owner, uint64_t new_data) {
    // Ensure that the action is authorized by the owner
    require_auth(owner);

    // Find the record by the owner's primary key
    auto itr = container.find(owner.value);
    check(itr != records.end(), "Error: Owner does not exist in the composition.");

    // Modify the existing record with the new data
    records.modify(itr, owner, [&](auto& row) {
        row.data = new_data;
    });
}

modify action

Compared to emplace action, modify requires 3 parameters: iterator, payer and lambda expression

• itr - Itterator found from find method pointing to the owner
• owner - Allocate memory used for this action
• [&](auto& row) {row.data = new_data;} - Set data record with new value

Find Action

Find Record by Primary Key

auto itr = container.find(owner.value);

// Custom error handling if owner does not exist
check(itr != container.end(), "Error: Owner does not exist in the composition.");

Breakdown

• auto itr = container.find(owner.value) Define iterator for owner value
• itr != container.end() Check whether iterator is not empty which implies that owner was found

Modify Record

Modify Existing Record

container.modify(itr, _self, [&]( auto& u ) {
    u.data = new_data; // Update the data field with new value
});

Breakdown

• container.modify() is used to modify an existing record in the indexed composition.
• itr is the iterator pointing to the record to modify.
• _self denotes that the contract itself will pay for any RAM costs involved.
• auto& u provides a reference to the record being modified.
• u.data = new_data assigns the new data value to the existing record.

Add Custom Error Handling

Ensure that only valid modifications are made, and provide meaningful error messages to assist debugging.

Validation Before Modification

check(new_data > 0, "Error: Data value must be greater than zero.");

Breakdown

• check() ensures that the new data value is valid.
• new_data > 0 checks that the data value is positive.
• The error message helps identify incorrect input.