The combination of
indexed does not work. To understand why, see how event arguments are stored in the blockchain.
All transactions that are executed generate a transaction receipt, which contains a property called
logs. You can look up the receipt using
In the transaction receipt, all event arguments that are not
indexed are included in the
data property. This property can hold values of arbitrary length. However, event arguments that are
indexed are not stored in the
data property, but end up in
topics. This allows filters to work.
For fixed size arguments types this is ok. The size of the entries in
topics is long enough to hold all fixed types supported by Solidity. However,
string can have an arbitrary length. To still be able to store those values in a topic, Solidity creates a hash of the value, which ends up in the topic.
The upside is that filters can still work, since you can just create the hash of the value you're filtering for. The downside is that web3 is unable to decode the value, since that would require performing the hash function in reverse direction.
To solve this problem, you have two options. You can remove
indexed from the argument, so it is stored in the
data field and no longer hashed to become part of a topic. Or, you change the type of the argument, e.g. into
bytes32 or another. Which is appropriate depends on your case.