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I'm trying to store records in a contract that are supposed to have an arbitrary number of fields. For example, one record would have "bitrate", another "ISBN" etc., in addition to a number of fixed fields. It is natural to store these fields as a string => string mapping.

Now, the question is, how do I pass these fields to the contract, and how do I get them back? Even the latest version of Solidity, v0.8, doesn't permit using mappings in public contract functions. I'm thinking of sending strings and parsing them inside the contract, but surely there is a more "native" Solidity way using low-level functions?

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  • 1. What are you going to do with this data in the contract? 2. The examples that you gave fit well into bytes32 and they can be transferred to the contract as an array, which can be split into pairs inside
    – Mad Jackal
    Jan 12 '21 at 10:32
  • Create a setter and getting function setField(itemId, fieldName, value) and getField(itemId, fieldName) public string. However blockchain is not designed to be a storage of string data and it is quite inefficient, so unless this is a theoretical exercise it does not make much sense.
    – Mikko Ohtamaa
    Jan 12 '21 at 11:04
  • @MadJackal great idea, can you add it as an answer?
    – ulu
    Jan 12 '21 at 11:34
  • @MikkoOhtamaa good idea, but it's not atomic, I can end up with a half-filled record. And it's not my choice, unfortunately, it's my employer's idea.
    – ulu
    Jan 12 '21 at 11:37
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    You can also pass an array of keys and values if you want to write multiple fields once.
    – Mikko Ohtamaa
    Jan 12 '21 at 12:04
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The examples that you gave fit well into bytes32 and they can be transferred to the contract as an array, which can be split into pairs inside.

If you just want to transfer big data through a contract without processing it in it, then it is probably better to use a decentralized file system (IPFS, Ethereum Swarm) or cloud storage, and put a link to the data file in the blockchain.

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