TLDR:
  • Illustrates how to use subgraphs to index all the ERC-20 token balances for every account on Ethereum.
  • Demonstrates mapping and schema design to track token, account, and token balance data comprehensively.
  • Retrieves token details from an ERC-20 contract using the generated AssemblyScript code for the transfer() event.
  • Wraps up with sample GraphQL queries on Chainstack Subgraphs to explore indexed account balances.

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Introduction

Before the Bored Apes and decentralized loans, there was a time when people were obsessed with creating tokens for everything that moved. This phase turned the Ethereum ecosystem into the Wild West of the cryptocurrency world—with new tokens popping up faster than you can say “to the moon.” The main ingredient of this frenzy was an implementation standard for creating fungible tokens, the ERC-20. ERC-20 was designed to provide a consistent set of rules and standards for developers to follow when creating new tokens on the Ethereum blockchain. This made it easier for people to understand how to create new tokens and for users to understand how to interact with them. Since its introduction in 2015, the ERC-20 has become the most widely used standard for creating new tokens on the Ethereum blockchain. So, it means that if you have dabbled in Ethereum, chances are you have either used, created, or even hoarded tokens made using the ERC-20 standard, and this article is aimed at the hoarders… I mean HODLers of ERC-20 tokens. In this article, you will use subgraphs to index Ethereum accounts and their ERC-20 token balances.
Check out A beginner’s guide to getting started with The Graph and Indexing Uniswap data with Subgraphs to learn more about developing Subgraphs.

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The modus operandi

Indexing the ERC-20 token balances presents quite a bit of a challenge compared to other exercises using subgraphs. You see while trying to gather this data, you won’t be focusing on a particular account or a contract. Here, you are trying to get the details of all the accounts that house any and all ERC-20 tokens. So where do we start? We know that every ERC-20 token is controlled by a smart contract that implements the ERC-20 standard on the Ethereum blockchain. The ERC-20 standard defines a set of mandatory and optional functions that a contract must implement in order to be considered an ERC-20 token. Of the many mandatory functions that should be implemented while creating an ERC-20 token, the transfer() function directly affects the token balance of an account.
Here’s a complete overview of the ERC-20 token standard: OpenZeppelin ERC-20 Doc
The transfer() function facilitates the transfer of tokens from one account to another. By mandate, the function must also emit a Transfer event that carries the details of the accounts involved in the transfer and the value of the tokens that were transferred. By capturing and processing the Transfer events that were emitted, we could access all the above-mentioned transfer information plus the token’s address (contract). With all data, I say we could create a nice index of token balances. So, let’s get to it.

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Prerequisites

Before we start scanning the Ethereum network for token balances, make sure you have installed the following on your computer:
  • Node (version ≥ 16) and the corresponding npm
  • A reasonably useful code editor
To develop and deploy subgraphs, you also need the graph-cli package (v0.51.2 or greater). To install this package on your computer, open a terminal and use the following command: 
npm install -g @graphprotocol/graph-cli

#for the specific package

npm install -g @graphprotocol/graph-cli@0.51.2
That takes care of all the requirements; now, let’s set up a subgraph project.

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Setting up the project

To write the code used in this article, the following Graph protocol repository was referred to:graphprotocol/erc20-subgraph.
To quickly spin up a subgraph project:
  1. Create a new directory.
  2. Open a terminal in the directory.
  3. Use the following command:
graph init
In this specific example, I used the ApeCoin smart contract; this means we’ll index all of the ApeCoin transfers that ever happened. You can use any Ethereum ERC-20 smart contract you want.
Also note that the graph-cli will pick up the beginning block number automatically, but you can edit it to start indexing from the block you want.
The command will prompt you for the following information: 
✔ Protocol · ethereum
✔ Product for which to initialize · subgraph-studio
✔ Subgraph slug · erc20-balance
✔ Directory to create the subgraph in · erc20-balance
? Ethereum network …
✔ Ethereum network · mainnet
✔ Contract address · 0x4d224452801ACEd8B2F0aebE155379bb5D594381
✔ Fetching ABI from Etherscan
✔ Fetching Start Block
✔ Start Block · 14204533
✔ Contract Name · ERC20
✔ Index contract events as entities (Y/n) · true
  Generate subgraph
  Write subgraph to directory
✔ Create subgraph scaffold
✔ Initialize networks config
✔ Initialize subgraph repository
✔ Install dependencies with yarn
✔ Generate ABI and schema types with yarn codegen
Add another contract? (y/n): n
Subgraph erc20-balance created in erc20-balance
Here, as you can see, when it comes to providing the contract address for the project, you can use any given ERC-20 token contract address. This is due to the fact that the transfer() function and the associated event are mandatory, meaning that every ERC-20 token contract would have to implement them, and they would do so in a uniform format. The graph-cli uses the contract address to fetch the contract ABI, which is required for accessing the contract functions. Since every ERC-20 token contract will have the transfer() function and the Transfer event, we can use the ABI of any given ERC-20 token contract for accessing them. You can get the token contract address from Etherscan. To avoid confusion, you can set the contract name as ERC20, as a nod to the generic nature in which we will be using its ABI.
Here’s a list of ERC-20 tokens to choose from Etherscan Token Tracker.
Once you provide all the information, the CLI tool will set up a well-structured project with some template code. 
.
├── abis
│   └── ERC20.json
├── networks.json
├── package.json
├── schema.graphql
├── src
│   ├── erc-20.ts
├── subgraph.yaml
├── tests
│   ├── erc-20.test.ts
│   └── erc-20-utils.ts
└── tsconfig.json
Here, you can see that inside the /abis directory, the contract ABI is saved as ERC20.json (based on the contract name that we provided).

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Writing the schema

Now that we have the base template for our project, we can start working on our schema file, schema.graphql. Within this file, we will define the data objects or entities we need. When you analyze our use case, you can see that in order to access the token balance, we also need information regarding the account and the token involved. Based on this requirement, let’s model our schema file: 
# Token details
type Token @entity {
  id: ID!
  #token name
  name: String!
  #token symbol
  symbol: String!
  #decimals used
  decimals: BigDecimal!
}

# account details
type Account @entity {
  #account address
  id: ID!
  #balances
  balances: [TokenBalance!]! @derivedFrom(field: "account")
}
# token balance details
type TokenBalance @entity {
  id: ID!
  #token
  token: Token!
  #account
  account: Account!
  #amount
  amount: BigDecimal!
}
Here, apart from the Token and Account entities, we have also modeled the token balance as a separate entity, TokenBalance. Since the token balance is associated with an account, we have declared a balances field inside the Account entity and declared it as a list of token balances; [TokenBalance!]. This represents the fact that a single account can have multiple token balances. The @derivedFrom directive in the field is used to declare it as a reverse lookup. By doing so, we have created a virtual field on the Account entity (Balances) that is derived from the relationship defined on the TokenBalance entity (account). Thus, the Balances field need not be set manually using the mapping file. Now that we are done with the schema let’s work on the manifest file.
Learn more about the schema by reading Explaining Subgraph schemas.

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Modifying the manifest

If you open the auto-generated manifest file subgraph.yaml in your code editor, you will see that it is populated with many details pertaining to the contract that we mentioned while setting up the project. For our project, we won’t be needing many of these details as most of them are specific to that contract. So, edit the file in the following way: 
specVersion: 0.0.5
schema:
  file: ./schema.graphql
dataSources:
  - kind: ethereum
    name: ERC20
    network: mainnet
    source:
      address: "0x4d224452801ACEd8B2F0aebE155379bb5D594381"
      abi: ERC20
      startBlock: 14204533
    mapping:
      kind: ethereum/events
      apiVersion: 0.0.7
      language: wasm/assemblyscript
      entities:
        - Token
        - Account
        - TokenBalance
      abis:
        - name: ERC20
          file: ./abis/ERC20.json
      eventHandlers:
        - event: Transfer(indexed address,indexed address,uint256)
          handler: handleTransfer
      file: ./src/erc-20.ts
In the updated manifest file, we have replaced the entities with the ones that we have defined. We have also removed all the eventHandlers except the one for the Transfer event and in the source section, we have removed the contract address, as our subgraph is not directed towards any particular address. We have also added the startBlock parameter in the source section. This will specify the block from which we wish to start the indexing. And with that, we have our new and improved manifest file.

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Code generation

Once you have the schema and the manifest files, you can use the following command to generate the required AssemblyScript code: 
graph codegen
This will generate an AssemblyScript class for every smart contract in the ABI files mentioned in our manifest file and also for its events. The generated code will be stored inside the /generated directory: 
.
├── abis
│   └── ERC20.json
├── generated
│   ├── ERC20
│   │   └── ERC20.ts
│   └── schema.ts
├── networks.json
├── package.json
├── schema.graphql
├── src
│   ├── erc-20.ts
├── subgraph.yaml
├── tests
│   ├── erc-20.test.ts
│   └── erc-20-utils.ts
└── tsconfig.json

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Creating the mappings

The mapping file is where we actually associate the blockchain data to the entities that we have defined in our schema. They contain the code for the eventHandlers, mentioned in our manifest file. As with the manifest file, if you open the auto-generated mapping file erc-20.ts inside the /src directory, you will find out that it contains the code for various eventHandlers, but we can remove most of them and work on the handleTransfer event handler. Now, before we start editing our core mapping file, I would like you to create a separate file in the /src directory. Within this file, we will be defining the code for:
  • Fetching the token details
  • Fetching the account details
  • Getting the token balance
You can include all these functions in the core mapping file itself, but keeping it in a separate file promotes modularity and makes the code more readable.
So, create a new file utils.ts inside our /src directory and add the following code: 
//import smart contract class from generated files
import { ERC20 } from "../generated/ERC20/ERC20";
//import entities
import { Account, Token } from "../generated/schema";
//import datatypes
import { BigDecimal, ethereum, Address } from "@graphprotocol/graph-ts";
//fetch token details
export function fetchTokenDetails(event: ethereum.Event): Token | null {
  //check if token details are already saved
  let token = Token.load(event.address.toHex());
  if (!token) {
    //if token details are not available
    //create a new token
    token = new Token(event.address.toHex());

    //set some default values
    token.name = "N/A";
    token.symbol = "N/A";
    token.decimals = BigDecimal.fromString("0");

    //bind the contract
    let erc20 = ERC20.bind(event.address);

    //fetch name
    let tokenName = erc20.try_name();
    if (!tokenName.reverted) {
      token.name = tokenName.value;
    }

    //fetch symbol
    let tokenSymbol = erc20.try_symbol();
    if (!tokenSymbol.reverted) {
      token.symbol = tokenSymbol.value;
    }

    //fetch decimals
    let tokenDecimal = erc20.try_decimals();
    if (!tokenDecimal.reverted) {
      token.decimals = BigDecimal.fromString(tokenDecimal.value.toString());
    }

    //save the details
    token.save();
  }
  return token;
}

//fetch account details
export function fetchAccount(address: string): Account | null {
  //check if account details are already saved
  let account = Account.load(address);
  if (!account) {
    //if account details are not available
    //create new account
    account = new Account(address);
    account.save();
  }
  return account;
}

//fetch the current balance of a particular token
//in the given account
export function fetchBalance(
  tokenAddress: Address,
  accountAddress: Address
): BigDecimal {
  let erc20 = ERC20.bind(tokenAddress); //bind token
  //set default value
  let amount = BigDecimal.fromString("0");
  //get balance
  let tokenBalance = erc20.try_balanceOf(accountAddress);
  if (!tokenBalance.reverted) {
    amount = tokenBalance.value.toBigDecimal();
  }
  return amount;
}
In this code, the fetchTokenDetails() gets us the details of a token. It does so by accessing the token address from the event parameter and binding it with the ERC20 (token contract ABI) class. This will let us access all the public, read-only functions from the token contract, and using those functions we can retrieve the details like token name, symbol, and decimals. The fetchAccount() function fetches the details of the account. The fetchBalance() function retrieves the balance of a particular token in a given account. To fetch the balance, the function uses the token address, which is given as a function parameter, and binds it with the ERC20 class. This allows you to access the balanceOf() function, which takes the account address as the parameter and returns the token balance. The balanceOf() is a read-only function that is implemented as part of the ERC-20 token standard. In the code, we also use the graph-ts library to import certain useful data types. This library is automatically installed when we set up the project. Once you add the new code, your project structure should look like this: 
├── abis
│   └── ERC20.json
├── generated
│   ├── ERC20
│   │   └── ERC20.ts
│   └── schema.ts
├── networks.json
├── package.json
├── schema.graphql
├── src
│   ├── erc-20.ts
│   └── utils.ts
├── subgraph.yaml
├── tests
│   ├── erc-20.test.ts
│   └── erc-20-utils.ts
└── tsconfig.json
Now, let’s work on our core mapping file /src/erc-20.ts.

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Handling Transfer event

Within the mapping file, you will be writing the code for the handleTransfer event handler. The code should be defined as a function of the same name, handleTransfer(): 
//import event class from generated files
import {Transfer} from "../generated/ERC20/ERC20"
//import entities
import {TokenBalance} from "../generated/schema"
//import the functions defined in utils.ts
import {
  fetchTokenDetails,
  fetchAccount,
  fetchBalance
} from "./utils"
//import datatype
import { BigDecimal} from "@graphprotocol/graph-ts";

export function handleTransfer(event: Transfer): void {
    let token = fetchTokenDetails(event);
    if (!token) { //if token == null
        return
      }

    //get account addresses from event
    let fromAddress = event.params.from.toHex();
    let toAddress = event.params.to.toHex();

    //fetch account details
    let fromAccount = fetchAccount(fromAddress);
    let toAccount = fetchAccount(toAddress);

    if (!fromAccount || !toAccount) {
    return;
    }

    //setting the token balance of the 'from' account
    let fromTokenBalance = TokenBalance.load(token.id + "-" + fromAccount.id);
    if (!fromTokenBalance) { //if balance is not already saved
					//create a new TokenBalance instance
					// while creating the new token balance,
					// the combination of the token address
					// and the account address is
					// passed as the identifier value
          fromTokenBalance = new TokenBalance(token.id + "-" + fromAccount.id);
          fromTokenBalance.token = token.id;
          fromTokenBalance.account = fromAccount.id;
    }

    fromTokenBalance.amount = fetchBalance(event.address,event.params.from)
		//filtering out zero-balance tokens - optional
    if(fromTokenBalance.amount != BigDecimal.fromString("0")){
      fromTokenBalance.save();
    }

    //setting the token balance of the 'to' account
    let toTokenBalance = TokenBalance.load(token.id + "-" + toAccount.id);
    if (!toTokenBalance) {
        toTokenBalance = new TokenBalance(token.id + "-" + toAccount.id);
        toTokenBalance.token = token.id;
        toTokenBalance.account = toAccount.id;
      }
    toTokenBalance.amount = fetchBalance(event.address,event.params.to)
    if(toTokenBalance.amount != BigDecimal.fromString("0")){
      toTokenBalance.save();
    }
}
The handleTransfer() function takes the Transfer event as the parameter. From the event, the code retrieves the address of the token and the addresses of accounts involved in the transfer. These addresses are used to fetch the details of the token and the accounts respectively. For retrieving the details, we make use of the functions that we defined in the utils.ts file. Once we have all the information, we use it to set the token balances of both accounts involved in the transfer. With that, we have everything that we need to deploy our subgraph, so let’s build our code and deploy the subgraph using Chainstack.

​
Building the code

To build the code, open a terminal in the root directory of the project and type: 
graph codegen &&  graph build
The command will compile your code and create corresponding WebAssembly files of .wasm format. The build outputs will be stored inside the /build directory. Now, let’s deploy the subgraph.

​
Deploying the subgraph

To deploy your subgraph:
  1. Head over to Chainstack.
  2. Create a new project.
  3. Go to the Subgraphs section.
  4. Click Add subgraph. The Add subgraph page is displayed.
  5. In the Create subgraph section:
    • Enter a Name for the subgraph
    • Select the Project you created
    • Click Add subgraph. The details page of the new subgraph is displayed.
Once the subgraph is deployed, on the subgraph details page:
  1. Scroll down to the part where it shows the Subgraph deployment command.
  2. Copy the command.
  3. Open a terminal in your project directory.
  4. Paste and run the deployment command.
The command will prompt you for the version labeling, and once you provide that, the command will automatically deploy your subgraph. Once the subgraph is deployed, the command will give you the GraphQL UI URL to interact with your subgraph.
Given the scale of the data that we are trying to index, the subgraph can take multiple hours/days to synchronize.

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Querying the data

The GraphQL UI URL provides a neat interface for querying and viewing the data. For fetching the list of all the accounts and their token balances, use the following query: 
{
  accounts {
    id
    balances {
      id
      token {
        id
        name
        symbol
        decimals
      }
      amount
    }
  }
}
This command will return all of the parameters like the following example: 
{
  "data": {
    "accounts": [
      {
        "id": "0x0000000000000000000000000000000000000000",
        "balances": []
      },
      {
        "id": "0x000000000000084e91743124a982076c59f10084",
        "balances": [
          {
            "id": "0x4d224452801aced8b2f0aebe155379bb5d594381-0x000000000000084e91743124a982076c59f10084",
            "token": {
              "id": "0x4d224452801aced8b2f0aebe155379bb5d594381",
              "name": "ApeCoin",
              "symbol": "APE",
              "decimals": "18"
            },
            "amount": "1"
          }
        ]
      }
}
Subgraphs use GraphQL as their query language. GraphQL has several built-in features you can use to customize your queries. Here are some additional parameters you can use: Pagination. Use the first and skip parameters to limit the number of results your query returned and paginate through the results:
  • first — specifies the maximum number of results to return.
  • skip — specifies the number of results to skip.
For example, to get the first 10 accounts, you would use: 
{
  accounts(first: 10) {
    id
    balances {
      id
      token {
        id
        name
        symbol
        decimals
      }
      amount
    }
  }
}
And to get the next 10 accounts, you would use: 
{
  accounts(first: 10, skip: 10) {
    id
    balances {
      id
      token {
        id
        name
        symbol
        decimals
      }
      amount
    }
  }
}
Ordering. You can sort your results using the orderBy and orderDirection parameters.
  • orderBy — specifies the field by which to sort the results.
  • orderDirection — specifies the direction of the sort, either asc for ascending or desc for descending.
For example, to get the accounts sorted by id in ascending order: 
{
  accounts(orderBy: id, orderDirection: asc) {
    id
    balances {
      id
      token {
        id
        name
        symbol
        decimals
      }
      amount
    }
  }
}
And that is how you use subgraphs to index all the token balances of Ethereum accounts.

​
Conclusion

Chainstack Subgraphs is a powerful tool that allows Web3 developers to index and query data from decentralized networks like Ethereum. They allow developers to easily access and use the data stored on these networks in their decentralized applications without building and maintaining their own infrastructure for data indexing and querying. This can save developers significant time and resources and make BUIDL-efficient DApps easier. The aim of this article was to showcase the scale of these subgraphs by using them to easily fetch, process, and organize data that is as scattered as ERC-20 token balances.

​
See also

A beginner’s guide to getting started with The Graph

Explaining Subgraph schemas

Debugging subgraphs with a local Graph Node

Deploying a Lido subgraph with Chainstack

Indexing Uniswap data with Subgraphs

Fetching subgraph data using JS

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About the author

44131280

Sethu Raman Omanakuttan

Developer Advocate @ Chainstack BUIDLs on Ethereum, NEAR , Graph Protocol and Oasis. Majored in computer science and technology.