chadwithuhc/ORMs.md

## ORMs.md

      
    Raw
  

              ORMs.md
            
          
    ORM


Object Relational Mapper


Object ... We're working with objects (as in database rows)
Relational ... Specifically handling relational data
Mapper ... Mapping objects together for you

Objectives


Describe the features of an ORM
Define common ORM terms
Describe common use cases for an ORM
Describe what ORM code is doing


ORM Features

ORMs generally include:


Easy querying without writing SQL (a.k.a. Query Building)

.find(), .where(), .orderBy()


Extending functionality with hooks or events

Validations, attribute manipulations, computed properties, limited scopes


Relationship handling without JOINs

Mapping relationships in model definitions
Easily loading relationships through Query Building


ORM Terms

Model ... Single row or record
Collection ... Group of models
Query Building ... Chaining methods together to generate a SQL query
Eager Loading ... Loading relationships when querying

ORM Use Cases


You are using relational data in your database
You would like to abstract your SQL query creation
You need to create SQL queries on-the-fly
You are too lazy to write SQL


Deep Thought


When would an ORM not be useful?


Relationships

Types


One-to-one
One-to-many
Many-to-many

Rules


One items have singular names
Many items have plural names
Tables are always named plural


One-to-one


One-to-one

// Get a book with specific ISBN-13 number, including its summary
new Book({'ISBN-13': '9780440180296'})
  .fetch({
    withRelated: ['summary']
  })
  .then(function(book) {
    // 'Slaughterhouse Five'
    book.get('title')
    // Access to properties on "Summary" model
    book.related('summary')
  })
In this code:

Fetching a book by ISBN-13 number
Eager Loading the summary data
Accessing properties on book model
Accessing related summary model


One-to-many


One-to-many

// Get a book with specific ISBN-13 number, including its pages
new Book({'ISBN-13': '9780440180296'})
  .fetch({
    withRelated: ['pages']
  }).then(function(book) {
    // Collection of page models
    book.related('pages')
    // Only retrieve odd pages
    book.related('pages').filter(function(page) {
      return page.get('number') % 1
    })
  })
In this code:

Fetching a book by ISBN-13 number
Eager Loading the pages
Accessing related pages Collection
??


One-to-many

// Get a book with specific ISBN-13 number, including its pages
new Book({'ISBN-13': '9780440180296'})
  .fetch({
    withRelated: ['pages']
  }).then(function(book) {
    // Collection of page models
    book.related('pages')
    // Only retrieve odd pages
    book.related('pages').filter(function(page) {
      return page.get('number') % 1
    })
  })
In this code:

Fetching a book by ISBN-13 number
Eager Loading the pages
Accessing related pages Collection
Filtering to only odd pages


Question


What is the following code doing?

Post.where({ id: 12345 }).fetch({
  withRelated: ['blog', 'author', 'comments']
}).then(function(post) {
  post.related('author').get('name')
  post.related('blog').get('title')
  post.related('comments').length
})

Many-to-many


Many-to-many

// Get author info along with their books
Author.where({name: 'George Costanza'})
  .fetch({
    withRelated: ['books']
  }).then(function(author) {
    author.related('books').where({ year: 2017 })
  })
In this code:

Fetching an author by name
??
??


Many-to-many

// Get author info along with their books
Author.where({name: 'George Costanza'})
  .fetch({
    withRelated: ['books']
  }).then(function(author) {
    author.related('books').where({ year: 2017 })
  })
In this code:

Fetching an author by name
Eager Loading their books
??


Many-to-many

// Get author info along with their books
Author.where({name: 'George Costanza'})
  .fetch({
    withRelated: ['books']
  }).then(function(author) {
    author.related('books').where({ year: 2017 })
  })
In this code:

Fetching an author by name
Eager Loading their books
Filtering their books down to releases this year


Question


What kind of relationship does the following code setup?

let Classroom = bookshelf.Model.extend({
  tableName: 'classrooms',
  students: function() {
    return this.hasMany(Student);
  }
});

let Student = bookshelf.Model.extend({
  tableName: 'students',
  classroom: function() {
    return this.belongsTo(Classroom);
  }
});

ORM Assessment


What does ORM stand for?
What are some common ORM features?
What is Eager Loading?

And finally ...

ORM Assessment


What is wrong with this code?

let Folders = bookshelf.Model.extend({
  tableName: 'folder',
  files: function() {
    return this.hasMany(File);
  }
});

let File = bookshelf.Model.extend({
  tableName: 'files',
  folders: function() {
    this.belongsTo(Folder);
  }
});

??
??
??
??


ORM Assessment


What is wrong with this code?

let Folders = bookshelf.Model.extend({
  tableName: 'folder',
  files: function() {
    return this.hasMany(File);
  }
});

let File = bookshelf.Model.extend({
  tableName: 'files',
  folders: function() {
    this.belongsTo(Folder);
  }
});

Folders model should be singular
??
??
??


ORM Assessment


What is wrong with this code?

let Folders = bookshelf.Model.extend({
  tableName: 'folder',
  files: function() {
    return this.hasMany(File);
  }
});

let File = bookshelf.Model.extend({
  tableName: 'files',
  folders: function() {
    this.belongsTo(Folder);
  }
});

Folders model should be singular
Folder model should have plural table name
??
??


ORM Assessment


What is wrong with this code?

let Folders = bookshelf.Model.extend({
  tableName: 'folder',
  files: function() {
    return this.hasMany(File);
  }
});

let File = bookshelf.Model.extend({
  tableName: 'files',
  folders: function() {
    this.belongsTo(Folder);
  }
});

Folders model should be singular
Folder model should have plural table name
File model should have folder property (singular)
??


ORM Assessment


What is wrong with this code?

let Folders = bookshelf.Model.extend({
  tableName: 'folder',
  files: function() {
    return this.hasMany(File);
  }
});

let File = bookshelf.Model.extend({
  tableName: 'files',
  folders: function() {
    this.belongsTo(Folder);
  }
});

Folders model should be singular
Folder model should have plural table name
File model should have folder property (singular)
return missing from File model


Objectives Complete!


Describe the features of an ORM
Define common ORM terms
Describe common use cases for an ORM
Describe what ORM code is doing