intrn(validator): make validator to check if role is only one related

server/validators/manager/is_the_only_role_to_any_user.ts

@@ -0,0 +1,41 @@
+import type { Request } from "!/types/dependent"
+import type { ValidationState, ValidationConstraints } from "!/types/validation"
+
+import Manager from "%/managers/role"
+
+/**
+ * Validator to check if role is the only role of any user.
+ *
+ * Note: The validator only works for resources with numerical role IDs only.
+ */
+export default async function(
+	currentState: Promise<ValidationState>,
+	constraints: ValidationConstraints<Request>
+): Promise<ValidationState> {
+	const state = await currentState
+
+	if (state.maySkip) return state
+
+	const manager = new Manager(constraints.request)
+
+	const modelID = state.value
+	const isTheOnlyRoleToAnyUser = await manager.isTheOnlyRoleToAnyUser(modelID)
+
+	if (isTheOnlyRoleToAnyUser) return state
+
+	const error = {
+		"field": constraints.field,
+		"friendlyName": constraints.friendlyName,
+		"messageMaker": (
+			field: string,
+			value: string
+		) => {
+			const subject = `The "${field}" with a value of "${value}"`
+			const predicate = "should have a user which is also have other rules after deletion."
+
+			return `${subject} ${predicate}`
+		}
+	}
+
+	throw error
+}