Can we check double creation of a state in Corda? - blockchain

Question

In my use case, I have a student enrolling to a specific program which in turn creates a student state on the ledger.
Now, if the same student enrolls with same credentials again, I want to avoid it and throw some exception or message.
One solution, I can think of is, I can query the vault before creating the student state and if that student is not found in the ledger than only he is allowed to enroll.
But, this seems like a vague idea.
Can someone provide a better approach or some other way I am not aware of?

Answer 1

You should implement schema for your state as in this example:
class CashState(
val owner: AbstractParty,
val pennies: Long) : ContractState, QueryableState {
override val participants get() = listOf(owner)
override fun generateMappedObject(schema: MappedSchema): PersistentState {
return when (schema) {
is CashSchemaV1 -> CashSchemaV1.PersistentCashState(
this.owner,
this.pennies
)
else -> throw IllegalArgumentException("Unrecognised schema $schema")
}
}
override fun supportedSchemas(): Iterable<MappedSchema> = listOf(CashSchemaV1)
}
The schema itself:
object CashSchema
#CordaSerializable
object CashSchemaV1 : MappedSchema(schemaFamily = CashSchema.javaClass, version = 1, mappedTypes = listOf(PersistentCashState::class.java)) {
#Entity
#Table(name = "contract_cash_states")
class PersistentCashState(
#Column(name = "owner_name", unique=true, nullable = true)
var owner: AbstractParty?,
#Column(name = "pennies", nullable = false)
var pennies: Long
) : PersistentState()
}
The key point is to make the columns unique, so when you add duplicate value the exception is thrown.
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