SubQuery powers the next generation of Polkadot dApps by allowing developers to extract, transform and query blockchain data in real time using GraphQL. In addition to this, SubQuery provides production quality hosting infrastructure to run these projects in.

This subquery example indexes the timestamp of each finalized block and it is an example of a CallHandler. By processing the timestamp.set extrinsic, and extracting the first arguments of it, we can retrieve the timestamp.

git clone https://github.com/subquery/subql-helloworld.git

cd subql-helloworld
yarn

yarn codegen

yarn build

docker-compose pull & docker-compose up

Open http://localhost:3000/ on your browser

query{
  starterEntities(last:5, orderBy:BLOCK_HEIGHT_ASC){
    nodes{
      blockHeight
    }
  }
}

As mentioned above, this project has a function called handleBlock. It uses a BlockHandler which is defined in the manifest file (project.yaml) as "kind: substrate/BlockHandler"

The schema.graphql file defines the variables blockHeight which is mandatory and of type Int.

If we examine the function handleBlock in more detail, you can see that this function takes one argument of type SubstrateBlock. It then creates a new instance of StarterEntity passing in the block.block.header.hash argument as a string and assigning this to the variable record.

Next, the blocknumber is converted to a number via toNumber() and assigned to record.blockHeight which is a StarterEntity meaning that the fields within are accessed with a dot.