intrn(validator): make validator to ensure there are no user

database/managers/role.ts

@@ -1,8 +1,8 @@
 import type { Pipe } from "$/types/database"
 import type { Serializable } from "$/types/general"
 import type { RoleQueryParameters } from "$/types/query"
-import type { Model as BaseModel, ModelCtor, FindAndCountOptions } from "%/types/dependent"
 import type { RoleAttributes, RoleResourceIdentifier } from "$/types/documents/role"
+import type { Model as BaseModel, ModelCtor, FindAndCountOptions } from "%/types/dependent"
 
 import User from "%/models/user"
 import Model from "%/models/role"
@@ -12,7 +12,6 @@ import AttachedRole from "%/models/attached_role"
 import RoleTransformer from "%/transformers/role"
 
 import Condition from "%/helpers/condition"
-import trimRight from "$/string/trim_right"
 import makeUnique from "$/array/make_unique"
 import segregateIDsExistentially from "%/helpers/segregate_IDs_existentially"
 

server/validators/manager/do_not_have_any_owned_user.ts

@@ -0,0 +1,43 @@
+import type { Request } from "!/types/dependent"
+import type { ValidationState, ValidationConstraints } from "!/types/validation"
+import type { DepartmentIdentifierListDocument } from "$/types/documents/department"
+
+import Manager from "%/managers/department"
+
+/**
+ * Validator to check if department still has user.
+ *
+ * Note: The validator only works for resources with numerical department IDs only.
+ */
+export default async function(
+	currentState: Promise<ValidationState>,
+	constraints: ValidationConstraints<Request>
+): Promise<ValidationState> {
+	const state = await currentState
+
+	if (state.maySkip) return state
+
+	const manager = new Manager(constraints.request)
+
+	const modelID = state.value
+	const departments = await manager.countUsers([ modelID ]) as DepartmentIdentifierListDocument
+	const hasNoUser = departments.data[0].meta.userCount === 0
+
+	if (hasNoUser) return state
+
+	const error = {
+		"field": constraints.field,
+		"friendlyName": constraints.friendlyName,
+		"messageMaker": (
+			field: string,
+			value: string
+		) => {
+			const subject = `The "${field}" with a value of "${value}"`
+			const predicate = "should have no existing user."
+
+			return `${subject} ${predicate}`
+		}
+	}
+
+	throw error
+}