Code shared from the Rust Playground

playground.rs

#![cfg_attr(not(feature = "std"), no_std)]

use ink_lang as ink;

#[ink::contract]
mod mycontract {
    use ink_storage::traits::{SpreadAllocate, SpreadLayout};

    #[derive(Debug, SpreadLayout, SpreadAllocate, Default)]
    #[cfg_attr(
        feature = "std",
        derive(
            scale_info::TypeInfo,
            ink_storage::traits::StorageLayout
        )
    )]
    pub struct Contract {
        map: ink_storage::Mapping<AccountId, u32>
    }

    #[ink(storage)]
    #[derive(SpreadAllocate)]
    pub struct MyContract {
        // Store a mapping from AccountIds to a u32
        map2: Contract,
    }

    impl MyContract {
        #[ink(constructor)]
        pub fn new() -> Self {
            // This call is required in order to correctly initialize the
            // `Mapping`s of our contract.
            // ink_lang::utils::initialize_contract(|contract: &mut Self| {
            //     let caller = Self::env().caller();
            //     contract.map.insert(&caller, &count);
            // })

            Self { map2: Default::default() }
        }

        // #[ink(constructor)]
        // pub fn default() -> Self {
            // Even though we're not explicitly initializing the `Mapping`,
            // we still need to call this
            // ink_lang::utils::initialize_contract(|_| {})
        // }

        // Grab the number at the caller's AccountID, if it exists
        #[ink(message)]
        pub fn get(&self) -> u32 {
            let caller = Self::env().caller();
            // self.map2.get(&caller).unwrap_or_default()
            self.map2.map.get(&caller).unwrap_or_default()
        }
    }
}