intrn(validator): validate tag is unused

Co-authored-by: Kenneth Trecy Tobias <19201.tobias.kennethtrecy.c@gmail.com>

server/validators/manager/has_no_other_posts.ts

@@ -0,0 +1,48 @@
+import type { Request } from "!/types/dependent"
+import type { ValidationState, ValidationConstraints } from "!/types/validation"
+
+import Manager from "%/managers/tag"
+
+/**
+ * Validator to check if a tag has no other tag activated.
+ *
+ * Note: The validator only works for resources with numerical tag 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 hasPosts = await manager.findWithID(Number(modelID), {
+		"constraints": {
+			"filter": {
+				"existence": "exists",
+				"mustHavePost": true
+			}
+		}
+	})
+
+	if (hasPosts) return state
+
+	const error = {
+		"field": constraints.field,
+		"friendlyName": constraints.friendlyName,
+		"messageMaker": (
+			field: string,
+			value: string
+		) => {
+			const subject = `The tag with an ID of "${value}"`
+			const predicate = "must be unused by any posts."
+
+			return `${subject} ${predicate}`
+		}
+	}
+
+	throw error
+}