Query API

[zyxw59]

I need an API on the Collection trait for searching for an object on field(s) other than id.

For the design of this API, I want to balance

• Expressiveness
• Ergonomics (of both the Rust API and how it would be exposed in a REST API)
• Ease of implementation (across multiple backends)
• Typedness
• Extensibility

[zyxw59]

For typedness, I could have each queryable type provide an enum of all its queryable fields — e.g. for Account it'd be something like

pub enum AccountQuery {
    Name(String),
    Description(String),
}

(and then maybe there's also a type for combining queries and applying modifiers like inequalities)

But then translating all of those enums to (e.g.) mongodb queries is a lot of work

[zyxw59]

ooh, for Transactions this gets hairier. A transaction has a Map<Id<Account>, Amount>, which has many possible queries one could imagine:

• any amount in the transaction = $x
• transaction involves account a
• transaction involves account a with amount $x
• gross value of transaction (i.e. sum of all positive values) = $x
• sum of amounts on accounts a, b, and c = $x

[zyxw59]

and then there's even more complication when adding comparators other than = into the mix — for equals, it's fine to translate "transaction involves account 123 with amount $4.56`" into

{ amounts: { $eq: { account: 123, value: 4.56 } } }

but if we want to search on the account and the value, with a different comparator (e.g. <), it needs to be

{ amounts: { account: 123, value: { $lt: 4.56 } } }

and I don't know of a good way to express this in the general case