fix(belongsTo): ignore null bindings when eagerloading

eagerloading will return null without reaching database, since null bindings throws exception

src/Lucid/Relations/BelongsTo.js

@@ -21,6 +21,26 @@ const CE = require('../../Exceptions')
  * @constructor
  */
 class BelongsTo extends BaseRelation {
+  /**
+   * Returns the first row for the related model
+   *
+   * @method first
+   *
+   * @return {Object|Null}
+   */
+  first () {
+    if (!this.parentInstance.$persisted) {
+      throw CE.RuntimeException.unSavedModel(this.parentInstance.constructor.name)
+    }
+
+    if (!this.$primaryKeyValue) {
+      return null
+    }
+
+    this._decorateQuery()
+    return this.relatedQuery.first()
+  }
+
   /**
    * Map values from model instances to an array. It is required
    * to make `whereIn` query when eagerloading results.

test/unit/lucid-belongs-to.spec.js

@@ -9,6 +9,7 @@
  * file that was distributed with this source code.
 */
 
+require('../../lib/iocResolver').setFold(require('@adonisjs/fold'))
 const test = require('japa')
 const fs = require('fs-extra')
 const path = require('path')
@@ -634,4 +635,104 @@ test.group('Relations | Belongs To', (group) => {
     assert.equal(userQuery.sql, helpers.formatQuery('select * from "users" where "id" in (?)'))
     assert.deepEqual(userQuery.bindings, helpers.formatBindings([1]))
   })
+
+  test('do not load relation with null value in foreign key', async (assert) => {
+    class User extends Model {
+    }
+
+    class Car extends Model {
+      user () {
+        return this.belongsTo(User)
+      }
+    }
+
+    User._bootIfNotBooted()
+    Car._bootIfNotBooted()
+
+    let userQuery = null
+    let carQuery = null
+    User.onQuery((query) => (userQuery = query))
+    Car.onQuery((query) => (carQuery = query))
+
+    await ioc.use('Database').table('users').insert({ username: 'virk' })
+    await ioc.use('Database').table('cars').insert({ name: 'E180', model: 'Mercedes', user_id: null })
+
+    const car = await Car.query().select(['id', 'name', 'user_id']).where('id', 1).first()
+    await car.load('user')
+    const json = car.toJSON()
+
+    assert.deepEqual(json, {
+      id: 1,
+      name: 'E180',
+      user: null,
+      user_id: null
+    })
+
+    assert.equal(userQuery, null)
+    assert.equal(carQuery.sql, helpers.formatQuery('select "id", "name", "user_id" from "cars" where "id" = ? limit ?'))
+    assert.deepEqual(carQuery.bindings, helpers.formatBindings([1, 1]))
+  })
+
+  test('do not eager load relation with null value in foreign key', async (assert) => {
+    class User extends Model {
+    }
+
+    class Car extends Model {
+      user () {
+        return this.belongsTo(User)
+      }
+    }
+
+    User._bootIfNotBooted()
+    Car._bootIfNotBooted()
+
+    let userQuery = null
+    let carQuery = null
+    User.onQuery((query) => (userQuery = query))
+    Car.onQuery((query) => (carQuery = query))
+
+    await ioc.use('Database').table('users').insert({ username: 'virk' })
+    await ioc.use('Database').table('cars').insert({ name: 'E180', model: 'Mercedes', user_id: null })
+
+    const car = await Car
+      .query()
+      .select(['id', 'name', 'user_id'])
+      .where('id', 1)
+      .with('user')
+      .first()
+
+    const json = car.toJSON()
+
+    assert.deepEqual(json, {
+      id: 1,
+      name: 'E180',
+      user: null,
+      user_id: null
+    })
+
+    assert.equal(userQuery, null)
+    assert.equal(carQuery.sql, helpers.formatQuery('select "id", "name", "user_id" from "cars" where "id" = ? limit ?'))
+    assert.deepEqual(carQuery.bindings, helpers.formatBindings([1, 1]))
+  })
+
+  test('throw exception when not eagerloading', async (assert) => {
+    class User extends Model {
+    }
+
+    class Car extends Model {
+      user () {
+        return this.belongsTo(User)
+      }
+    }
+
+    User._bootIfNotBooted()
+    Car._bootIfNotBooted()
+
+    await ioc.use('Database').table('users').insert({ username: 'virk' })
+    await ioc.use('Database').table('cars').insert({ name: 'E180', model: 'Mercedes', user_id: null })
+
+    const car = await Car.query().where('id', 1).first()
+    const fn = () => car.user().toSQL()
+    assert.throw(fn, 'E_UNSAVED_MODEL_INSTANCE: Cannot process relation, since Car model is not persisted to database or relational value is undefined')
+  })
 })