CODEWITHSUNDEEP
storagesubstrate
Phil H
Phil H

Reputation: 20141

Does Substrate do special things with Option types in Storage?

I understand that Substrate will return the 0 or null value when Storage is queried for a key that doesn't exist. Does it do something special with Option types?

I ask because in the default node-template, the storage item Something is declared as

Something: Option<u32>;

But later in the do_something setter, the method signature declares something: u32, but passes it directly without wrapping it in Some():

Something::put(something);

instead of 

Something::put(Some(something));

If Substrate is interpreting a call to store T as storing Some<T>, does that also mean if I store None in a map against a key then it will delete the key? Is it storing Some(T) or T?

Or is it just that Rust is building a specialised method call for Something::put(some: T) case as well as Something::put(some: Option<T>)?

Upvotes: 1

Views: 475

Answers (1)

gui
gui

Reputation: 516

Indeed decl_storage expand differently if the value is an Option or not.

The trait implemented is srml_support::storage::StorageValue

  • if the value is Option<MyType> the trait implemented is
srml_support::storage::StorageValue<MyType> {
    type Query = Option<MyType>;
}

in the implementation get will return None if no value is found in the trie at the key used by this storage.

  • if the value is not an option but whatever other type MyType the trait implemented is
srml_support::storage::StorageValue<MyType> {
    type Query = MyType;
}

in the implementation get will return Default if no value is found in the trie at the key used by this storage.

Note that this default can be overriden using the syntax:

NotOptionSomething: u32 = MyDifferentDefault;

If Substrate is interpreting a call to store T as storing Some, does that also mean if I store None in a map against a key then it will delete the key? Is it storing Some(T) or T?

At this point this is no longer substrate interpretation but just a rust trait implementation. The put function is implemented to store the encoded value in the trie. Not wrapped in an option.

Upvotes: 2

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