Domain.View reference

View

Source: packages/domain/src/View.ts

A read model: events folded into state per key, kept up to date as new events arrive. A view can emit events of its own, route one event to several keys, and decide who may read it.

View: Folding events into state

import * as Domain from "@alchemy.run/domain";
import * as Schema from "effect/Schema";
import { Account } from "./Account.ts";

export class AccountSummary extends Domain.View("AccountSummary", {
  from: [Account],
  key: Account,
  state: Schema.Struct({ balance: Schema.Int }),
  initial: { balance: 0 },
  evolve: {
    MoneyDeposited: (v, e) => ({ ...v, balance: e.balanceAfter }),
    MoneyWithdrawn: (v, e) => ({ ...v, balance: e.balanceAfter }),
  },
}) {}

View: Emitting events

import * as Domain from "@alchemy.run/domain";
import * as Schema from "effect/Schema";
import { Account } from "./Account.ts";

export class StatementLine extends Domain.Event("StatementLine", {
  kind: Schema.Literals(["deposit", "withdrawal"]),
  amount: Schema.Int,
}) {}

export class Statement extends Domain.View("Statement", {
  from: [Account],
  key: Account,
  events: [StatementLine],
  emit: {
    MoneyDeposited: (e) => new StatementLine({ kind: "deposit", amount: e.amount }),
    MoneyWithdrawn: (e) => new StatementLine({ kind: "withdrawal", amount: e.amount }),
  },
}) {}

View: Routing one event to several keys

keyOf: {
  TransferRequested: {
    out: ({ event }) => Account.ref(event.from),
    in: ({ event }) => Account.ref(event.to),
  },
},

View: Validating reads

middleware: [Session],
rejects: [NotOwner],
validate: Effect.fn(function* (_request, summary) {
  const actor = yield* CurrentActor;
  if (summary?.owner && summary.owner !== actor.customerId) return yield* new NotOwner();
}),