junction.include shim to get edge fields back onto the edge

junctionShim.js

/*
 * CONNECTION EDGE FIELDS SHIM
 * (c) 2017 Conduit Analytics Inc. All rights reserved. https://conduithq.com
 *
 * Temporary fix for https://github.com/stems/join-monster/issues/155
 * By Brandon Wang (https://github.com/bw)
 *
 * ## Why
 *
 * This shim hacks in the ability to request fields from a junction table to Relay-compliant edge fields directly.
 * The current limitation requires us to resolve on the node, so a "shim field" is used. The shim field must be
 * requested any time that edge fields are also requested. We serialize any columns we want to use from the junction
 * table into a JSON payload on the shim field itself.
 *
 * ## Usage
 *
 * Let's assume there are two tables, 'actors' and 'movies', and a junction table 'actor_movie_links' which has the
 * 'movie_role' that we want.
 * ```js
 * export const ActorMovieEdgeShim = new JunctionShim({
 *   columns: ['movie_role'],
 * });
 * ```
 *
 * Where the field is defined for usage, set junction.include equal to ActorMovieEdgeShim.junctionInclude().
 * Now you can request them on the Relay-compliant edge:
 * ```js
 * edgeFields: {
 *   movieRole: {
 *     type: GraphQLString,
 *     resolve: ActorMovieEdgeShim.resolve(({ movie_role }) => movie_role),
 *   }
 * }
 * ```
 */

import assert from 'assert';
import { GraphQLError } from 'graphql';

class JunctionShim {
  constructor(config = {}) {
    this.columns = config.columns || [];
    this.shimField = config.shimField || '_edgeFix';
  }

  /**
   * Constructs a pseudo-field with junction table columns. Set junction.include equal to this.
   * @returns {Object}
   */
  junctionInclude() {
    return {
      [this.shimField]: {
        sqlExpr: (junctionTable) => `json_build_object(${this.columns.map(
          (column) => `'${column}',${junctionTable}."${column}"`,
        ).join(',')})`,
      },
    };
  }

  /**
   * Runs the resolver passed into the parameter with the relevant fields as an argument.
   *
   * @param {Function} resolve: Resolver that accepts edge fields.
   * @returns {function({ edgeFields })}
   */
  resolve(resolve) {
    assert.ok(resolve, 'Missing resolving function as parameter.');

    return ({ node }) => {
      if (!node[this.shimField]) {
        throw new GraphQLError(
          `Temporary hack: In order to resolve edge fields, include the '${this.shimField}' field on the node itself.`);
      }

      return resolve(node[this.shimField]);
    };
  }
}

export default JunctionShim;