Contract Migration?

[Reecepbcups]

At the end just having a chapter on contract migration would be useful, took me a bit to learn how to do it via docs. ( This may be a good part-2 to #6 's chapter )

Could be as simple as this

pub struct ConfigResponse {
    pub name: String, // cw2 name
    pub version: String, // cw2 version from the .toml
}

pub const CONFIG = Item<ConfigResponse>

Then a migrate entry point which changes this value on upload

#[entry_point]
pub fn migrate(deps: DepsMut, _env: Env, _msg: MigrateMsg) -> Result<Response, ContractError> {
    let ver = cw2::get_contract_version(deps.storage)?;
    
    // ensure we are migrating from an allowed contract
    if ver.contract != CONTRACT_NAME {
        return Err(StdError::generic_err("Can only upgrade from same type").into());
    }
    
    if ver.version >= (*CONTRACT_VERSION).to_string() {
        return Err(StdError::generic_err("Cannot upgrade from a newer version").into());
    }

    // set the new version
    cw2::set_contract_version(deps.storage, CONTRACT_NAME, CONTRACT_VERSION)?;

    // state breaking migrations here

    // update the version field in the CONFIG since we have updated the contract
    let mut config = CONFIG.load(deps.storage)?;
    config.version = CONTRACT_VERSION.to_string();
    CONFIG.save(deps.storage, &config)?;

    Ok(Response::default()
        .add_attribute("action", "migration")
        .add_attribute("version", CONTRACT_VERSION)