directus/api/src/utils/get-relation-info.ts

import { Relation, RelationMeta } from '@directus/shared/types';
import { getRelationType } from './get-relation-type';

type RelationInfo = {
	relation: Relation | null;
	relationType: string | null;
};

export function getRelationInfo(relations: Relation[], collection: string, field: string): RelationInfo {
	if (field.startsWith('$FOLLOW') && field.length > 500) {
		throw new Error(`Implicit $FOLLOW statement is too big to parse. Got: "${field.substring(500)}..."`);
	}

	const implicitRelation = field.match(/^\$FOLLOW\((.*?),(.*?)(?:,(.*?))?\)$/)?.slice(1);

	if (implicitRelation) {
		if (implicitRelation[2] === undefined) {
			const [m2oCollection, m2oField] = implicitRelation;

			const relation: Relation = {
				collection: m2oCollection.trim(),
				field: m2oField.trim(),
				related_collection: collection,
				schema: null,
				meta: null,
			};

			return { relation, relationType: 'o2m' };
		} else {
			const [a2oCollection, a2oItemField, a2oCollectionField] = implicitRelation;

			const relation: Relation = {
				collection: a2oCollection.trim(),
				field: a2oItemField.trim(),
				related_collection: collection,
				schema: null,
				meta: {
					one_collection_field: a2oCollectionField.trim(),
				} as RelationMeta,
			};

			return { relation, relationType: 'o2a' };
		}
	}

	const relation =
		relations.find((relation) => {
			return (
				(relation.collection === collection && relation.field === field) ||
				(relation.related_collection === collection && relation.meta?.one_field === field)
			);
		}) ?? null;

	const relationType = relation ? getRelationType({ relation, collection, field }) : null;

	return { relation, relationType };
}