You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: on_chain.rs
    path of file: ./repos/fuel-core/crates/fuel-core/src/database/database_description
    
    the code of the file until where you have to start completion: use crate::database::database_description::DatabaseDescription;
use fuel_core_types::fuel_types::BlockHeight;

#[derive(Clone, Debug)]
pub struct OnChain;

impl DatabaseDescription for OnChain {
    type Column = fuel_core_sto
    
    fn prefix(column: &Self::Column) -> Option<usize> {
        match column {
            Self::Column::ContractsAssets | Self::Column::ContractsState => {
                // prefix is address length
                Some(32)
            }
            _ => None,
        }
    }