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What is a Smart Relationship?

Sometimes, you want to create a virtual relationship between two set of data that does not exist in your database. A concrete example could be creating a relationship between two collections available in two different databases. Creating a Smart Relationship allows you to customize with code how your collections are linked together.

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Create a BelongsTo Smart Relationship

On the Live Demo example, we have an order which belongsTo a customer which belongsTo a delivery address. We’ve created here a BelongsTo Smart Relationship that acts like a shortcut between the order and the delivery address. A BelongsTo Smart Relationship is created like a Smart Field with the reference option to indicate on which collection the Smart Relationship points to. You will also need to code the logic of the search query. 
const { collection } = require('forest-express-sequelize');
const models = require('../models');
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collection('orders', {
  fields: [{
    field: 'delivery_address',
    type: 'String',
    reference: 'addresses.id',
    get: function (order) {
      return models.addresses
        .findAll({
          include: [{
            model: models.customers,
            where: { id: order.customer_id },
            include: [{
              model: models.orders,
              where: { ref: order.ref }
            }]
          }],
        })
        .then((addresses) => {
          if (addresses) { return addresses[0]; }
        });
    }
  }]
});
Upon browsing, an API call is triggered when accessing the data of the HasMany relationships in order to fetch them asynchronously. In the following example, the API call is a GET on /products/:product_id/relationships/buyers.Option 1: using Sequelize ORMWe’ll use the findAll and count methods provided by Sequelize to find and count all customers who bought the current product (buyers).Then, you should handle pagination in order to avoid performance issue. The API call has a query string available which gives you all the necessary parameters you need to enable pagination.Finally, you don’t have to serialize the data yourself. The Forest agent already knows how to serialize your collection (customers in this example). You can access to the serializer through the recordsGetter.serialize function.
const express = require('express');
const {
  PermissionMiddlewareCreator,
  RecordSerializer,
} = require('forest-express-sequelize');
const { products, customers, orders } = require('../models');

const router = express.Router();
const permissionMiddlewareCreator = new PermissionMiddlewareCreator('products');

router.get(
  '/products/:product_id/relationships/buyers',
  (request, response, next) => {
    const productId = request.params.product_id;
    const limit = parseInt(request.query.page.size, 10) || 20;
    const offset = (parseInt(request.query.page.number, 10) - 1) * limit;
    const include = [
      {
        model: orders,
        as: 'orders',
        where: { product_id: productId },
      },
    ];

    // find the customers for the requested page and page size
    const findAll = customers.findAll({
      include,
      offset,
      limit,
    });

    // count all customers for pagination
    const count = customers.count({ include });

    // resolve the two promises and serialize the response
    const serializer = new RecordSerializer(customers);
    Promise.all([findAll, count])
      .then(([customersFound, customersCount]) =>
        serializer.serialize(customersFound, { count: customersCount })
      )
      .then((recordsSerialized) => response.send(recordsSerialized))
      .catch(next);
  }
);
Option2: using raw SQLWe’ll use raw SQL query and Sequelize to count and find all customers who bought the current product (buyers).Then, you should handle pagination in order to avoid performance issue. The API call has a query string available which gives you all the necessary parameters you need to enable pagination.Finally, you don’t have to serialize the data yourself. The Forest agent already knows how to serialize your collection (customers in this example). You can access to the serializer through the recordsGetter.serialize function.
const express = require('express');
const router = express.Router();
const models = require('../models');

router.get('/products/:product_id/relationships/buyers', (req, res, next) => {
  let limit = parseInt(req.query.page.size) || 10;
  let offset = (parseInt(req.query.page.number) - 1) * limit;
  let queryType = models.sequelize.QueryTypes.SELECT;

  let countQuery = `
    SELECT COUNT(*)
    FROM customers
    JOIN orders ON orders.customer_id = customers.id
    JOIN products ON orders.product_id = products.id
    WHERE product_id = ${req.params.product_id};
  `;

  let dataQuery = `
    SELECT customers.*
    FROM customers
    JOIN orders ON orders.customer_id = customers.id
    JOIN products ON orders.product_id = products.id
    WHERE product_id = ${req.params.product_id}
    LIMIT ${limit}
    OFFSET ${offset}
  `;

  const serializer = new RecordSerializer(customers);
  Promise.all([
    // Since support to multiple db connections was added you have to use the connection name defined in config/databases.js
    // here using default
    models.connections.default.query(countQuery, { type: queryType }),
    models.connections.default.query(dataQuery, { type: queryType }),
  ])
    .then(([count, queryResult]) =>
      serializer.serialize(queryResult[0], { count: count[0].count })
    )
    .then((serializedResult) => res.send(serializedResult))
    .catch((err) => next(err));
});

module.exports = router;
If your primary key column name (customer_id) is different than the model field name (customerId), you must alias the primary key column with the name of the model field in the dataQuery.

Ex: SELECT customers.*, customers.customer_id AS “customerId”