Build reverse allowances map during migration

contracts/cw20-base/src/contract.rs

@@ -1,5 +1,6 @@
 #[cfg(not(feature = "library"))]
 use cosmwasm_std::entry_point;
+use cosmwasm_std::Order::Ascending;
 use cosmwasm_std::{
     to_binary, Binary, Deps, DepsMut, Env, MessageInfo, Response, StdError, StdResult, Uint128,
 };
@@ -17,7 +18,10 @@ use crate::allowances::{
 use crate::enumerable::{query_all_accounts, query_owner_allowances, query_spender_allowances};
 use crate::error::ContractError;
 use crate::msg::{ExecuteMsg, InstantiateMsg, MigrateMsg, QueryMsg};
-use crate::state::{MinterData, TokenInfo, BALANCES, LOGO, MARKETING_INFO, TOKEN_INFO};
+use crate::state::{
+    MinterData, TokenInfo, ALLOWANCES, ALLOWANCES_SPENDER, BALANCES, LOGO, MARKETING_INFO,
+    TOKEN_INFO,
+};
 
 // version info for migration info
 const CONTRACT_NAME: &str = "crates.io:cw20-base";
@@ -598,7 +602,14 @@ pub fn query_download_logo(deps: Deps) -> StdResult<DownloadLogoResponse> {
 }
 
 #[cfg_attr(not(feature = "library"), entry_point)]
-pub fn migrate(_deps: DepsMut, _env: Env, _msg: MigrateMsg) -> Result<Response, ContractError> {
+pub fn migrate(deps: DepsMut, _env: Env, _msg: MigrateMsg) -> Result<Response, ContractError> {
+    // Build reverse map of allowances per spender
+    let data = ALLOWANCES
+        .range(deps.storage, None, None, Ascending)
+        .collect::<StdResult<Vec<_>>>()?;
+    for ((owner, spender), allowance) in data {
+        ALLOWANCES_SPENDER.save(deps.storage, (&spender, &owner), &allowance)?;
+    }
     Ok(Response::default())
 }