Using with aggregations fields

Views support make easy to work with aggregations fields, beig abble to use it as standar fields because this are going to be proccesed by the db engine.

Using the option param fieldsMap, you can to map a View field to an aggregation function.

Example using count

// Create User Model
class User extends Model {};

User.init({
  username: DataTypes.STRING,
  birthday: DataTypes.DATE,
}, { sequelize, modelName: 'user' });

// Create Post Model

class Post extends Model {};

Post.init({
  title: DataTypes.STRING,
  content: DataTypes.TEXT,
}, { sequelize, modelName: 'post' });

// Create Post -> User sssociation
Post.belongsTo(User, {allowNull: false});
User.hasMany(Post);

// Create View
class UserPosts extends View {}; 

UserPosts.init({
    username: DataTypes.STRING,
    posts: DataTypes.INTEGER,
}, {
    sequelize,
    modelName: 'user_posts',
    timestamps: false,
    viewQueryOptions: {
        model: User, 
        attributes: [
            'id',
            'username',
            [fn('count', 'post.id'), 'posts']
        ],
        group: ['user.id'],
        include: [{
            model: Post,
            attributes: [],
        }],
    },
});

(async () => {
  await sequelize.sync();

  const jane = await User.create({
    username: 'janedoe',
    birthday: new Date(1980, 6, 20),
  });

  const robert = await User.create({
    username: 'robert',
    birthday: new Date(1987, 1, 25),
  });

  const post1 = await Post.create({
    title: 'Post 1',
    content: 'This is the first post',
    userId: jane.id,
  });

  const post2 = await Post.create({
    title: 'Post 2',
    content: 'This is the second post',
    userId: robert.id,
  });

  const post3 = await Post.create({
    title: 'Post 3',
    content: 'This is the third post',
    userId: robert.id,
  });

  const result = await UserPosts.findAll();

  console.log(result.map((user) => user.toJSON()));
})();

This should to write out:

[
  { id: 1, username: 'janedoe', posts: 1 },
  { id: 2, username: 'robert', posts: 2 }
]

Using where/group/order with aggregated fields

The aesy way to make complex operations with aggregators is becouse you treat this like standar fields, being abble to do for ex:

const result = UserPost.findAll({
    attributes: ['username', 'posts'],
    where: {
      posts: {
        [Op.gte]: 10,
      },
    },
    order: [
      ['posts', 'DESC'],
    ],
});

In this case, we are filtering and ordering using the final count of posts, in a squelize simple query, this is very complex becaouse we need to group and define aggregators in the query. Using views, there are simplest.