Create a Smart relationship

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.
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 indicates on which collection the Smart Relationship points to.
SQL
Mongodb
/forest/orders.js
const Liana = require('forest-express-sequelize');
const models = require('../models');
​
Liana.collection('orders', {
  fields: [{
    field: 'delivery_address',
    type: 'String',
    reference: 'addresses.id',
    get: function (order) {
      return models.addresses
        .findAll({
          include: [{
            model: models.customers,
            include: [{
              model: models.orders,
              where: { ref: order.ref }
            }]
          }],
        })
        .then((addresses) => {
          if (addresses) { return addresses[0]; }
        });
    }
  }]
});
/forest/orders.js
const Liana = require('forest-express-mongoose');
const Address = require('../models/addresses');
​
Liana.collection('Order', {
  fields: [{
    field: 'delivery_address',
    type: 'String',
    reference: 'Address._id',
    get: function (order) {
      return Address
        .aggregate([
          {
            $lookup:
            {
              from: 'orders',
              localField: 'customer_id',
              foreignField: 'customer_id',
              as: 'orders_docs'
            }
          }, 
          {
            $match:
            {
              'orders_docs._id': order._id
            }
          }
        ])
        .then((addresses) => {
          if (addresses) { return addresses[0]._id; }
        });
    }
  }]
});
​
Create a HasMany Smart Relationship
On the Live Demo example, we have a product hasMany orders and an order belongsTo customer. We’ve created a Smart Relationship that acts like a shortcut: product hasMany customers.
A HasMany Smart Relationship is created like a Smart Field with the reference option to indicates on which collection the Smart Relationship points to.
SQL
Mongodb
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.
We’ve built the right SQL query using Sequelize to count and find all customers who bought the current product.
Then, you should handle pagination in order to avoid performance issue. The API call has a querystring 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 Liana already knows how to serialize your collection (customers in this example). You can access to the serializer through the Liana.ResourceSerializer object.
/forest/products.js
const Liana = require('forest-express-sequelize');
​
Liana.collection('products', {
  fields: [{
    field: 'buyers',
    type: ['String'],
    reference: 'customers.id'
  }]
});
/routes/products.js
const P = require('bluebird');
const express = require('express');
const router = express.Router();
const Liana = require('forest-express-sequelize');
const models = require('../models');
​
router.get('/products/:product_id/relationships/buyers', 
  Liana.ensureAuthenticated, (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}
    `;
​
    return P
      .all([
        models.sequelize.query(countQuery, { type: queryType }),
        models.sequelize.query(dataQuery, { type: queryType })
      ])
      .spread((count, customers) => {
        return new Liana.ResourceSerializer(Liana, models.customers, customers, null, {}, {
          count: count[0].count
        }).perform();
      })
      .then((products) => {
        res.send(products);
      })
      .catch((err) => next(err));
  });
​
module.exports = router;
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 /Product/:product_id/relationships/buyers.
We use the $lookup operator of the aggregate pipeline. Since there's a many-to-many relationship between Product and Customer, the $lookup operator needs to look into orders which is an array we have to flatten first using $unwind.
Finally, you don’t have to serialize the data yourself. The Forest Liana already knows how to serialize your collection (Customer in this example). You can access to the serializer through the Liana.ResourceSerializer object.
/forest/products.js
const Liana = require('forest-express-mongoose');
​
Liana.collection('products', {
  fields: [{
    field: 'buyers',
    type: ['String'],
    reference: 'Customer._id'
  }]
});
/routes/products.js
const P = require('bluebird');
const express = require('express');
const router = express.Router();
const Liana = require('forest-express-mongoose');
const Customers = require('../models/customers');
const mongoose = require('mongoose');
​
router.get('/Product/:product_id/relationships/buyers', 
  Liana.ensureAuthenticated, (req, res, next) => {
    let limit = parseInt(req.query.page.size) || 10;
    let offset = (parseInt(req.query.page.number) - 1) * limit;
​
    let countQuery = Customers.aggregate([
          {
            $lookup:
            {
              from: 'orders',
              localField: 'orders',
              foreignField: '_id',
              as: 'orders_docs'
            }
          },
          {
            $unwind: "$orders_docs"
          },
          {
            $lookup:
            {
              from: 'products',
              localField: 'orders_docs._id',
              foreignField: 'orders',
              as: 'products_docs'
            }
          },
          {
            $match:
            {
              'products_docs._id': mongoose.Types.ObjectId(req.params.product_id)
            }
          },
          {
            $count: "products_docs"
          }
        ]);
​
    let dataQuery = Customers.aggregate([
          {
            $lookup:
            {
              from: 'orders',
              localField: 'orders',
              foreignField: '_id',
              as: 'orders_docs'
            }
          },
          {
            $unwind: "$orders_docs"
          },
          {
            $lookup:
            {
              from: 'products',
              localField: 'orders_docs._id',
              foreignField: 'orders',
              as: 'products_docs'
            }
          },
          {
            $match:
            {
              'products_docs._id': mongoose.Types.ObjectId(req.params.product_id)
            }
          }
        ]);
​
    return P
      .all([
        countQuery,
        dataQuery
      ])
      .spread((count, customers) => {
        return new Liana.ResourceSerializer(Liana, Customers, customers, null, {}, {
          count: count.orders_count
        }).perform();
      })
      .then((products) => {
        res.send(products);
      })
      .catch((err) => next(err));
  });
​
module.exports = router;
​
You must return an id column in the Serializer. If this is not the case, you can add an id column to your SQL query using an alias.

Ex: SELECT customers.*, customers."customerId" AS id
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