To implement field whitelisting and access validation for dynamic SOQL construction in Salesforce, follow these steps:
1. **Field Whitelisting**:
- Create a whitelist of acceptable fields and objects.
- Use `Schema.getGlobalDescribe()` to retrieve a map of all objects and their fields.
- Compare user-supplied input against this whitelist to ensure only valid fields and objects are used in the query.
2. **Access Validation**:
- Validate that the user has the necessary permissions to access the specified object and fields before executing the query.
- Use `Schema.DescribeSObjectResult` and methods like `isAccessible()`, `isCreateable()`, `isUpdateable()`, and `isDeletable()` to enforce Field-Level Security (FLS) and CRUD permissions.
- For modern implementations, consider using `WITH USER_MODE` in SOQL queries or `AccessLevel.USER_MODE` for Database methods to automatically enforce user permissions.
3. **Safe SOQL Construction**:
- Use bind variables to include user-supplied input in the query. This prevents SOQL injection by treating user input as a parameter rather than executable code.
4. **Example Implementation**:
```apex
Schema.DescribeSObjectResult objectDesc = Schema.getGlobalDescribe().get(objectType).getDescribe();
if (!objectDesc.isAccessible()) {
throw new IllegalArgumentException('User does not have permission to access this object.');
}
List<String> validFields = new List<String>{'Name', 'CreatedDate'}; // Example whitelist
if (!validFields.contains(fieldName)) {
throw new IllegalArgumentException('Invalid field specified.');
}
String query = 'SELECT ' + fieldName + ' FROM ' + objectType + ' WHERE CreatedDate = TODAY WITH USER_MODE';
List<SObject> records = Database.query(query);
```
This ensures that only authorized fields and objects are queried, and user permissions are respected.