belongsTo Associations not getting mapped using ActiveModelAdapter


I am working on building an e-commerce site. My models and their associations are as follows-

App.Product = DS.Model.extend({
  // variables
  name: attr(),
  description: attr(),
  display_price: attr(),
  stock: attr(),
  dates: attr(),

  // associations
  taxons: DS.hasMany('taxon'),
  provider: DS.belongsTo('provider', { async: true }),
  images: DS.hasMany('image', { async: true }),
  variant: hasOne('variant'), // 

App.Variant = DS.Model.extend({
  // variables
  isMaster: attr(),
  name: attr(),
  images: attr(),

  // associations
  lineItems: DS.hasMany('lineItem'),
  product: DS.belongsTo('product')

App.LineItem = DS.Model.extend({
  // variables
  quantity: attr(),
  price: attr(),
  date: attr(),
  meal_type: attr(),
  total: attr(),
  display_amount: attr(),

  // associations
  variant: DS.belongsTo('variant'),
  cart: DS.belongsTo('cart')

I am loading all the product variants by sending an ajax request (using ActiveModelAdapter). Now, for adding the variants in the cart, I have an api end point which accepts variant_id and quantity. This end point responds with the whole Cart containing all the Line Items. When I add the variants from the variant detail view (by clicking ‘add’ button), the variant is added successfully and I show the Cart with the response. Now, for increasing the quantity of the added Line Item, I need to find the variant from the LineItem. To do so, I am doing the following-


This does not work. To my understanding, since variant belongsTo LineItem, this should have worked. Please help!