Rails 4敏捷Web开发指南：保持同步与实战

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"《敏捷Web开发与Rails 4th版》是一本专注于Rails 3版本的指南，由Ruby on Rails核心团队成员撰写，旨在帮助开发者理解和实践敏捷Web开发方法。作者强调了随着Rails不断更新，早期版本的应用可能与新版本存在兼容性问题，因此在阅读本书时，读者应确保安装了正确的Rails版本，如第1章所述的安装步骤（第24页）。书中提供了详实的实例，以便读者跟随学习。要确认当前运行的Rails版本，可以在命令行中输入`rails -v`。对于那些影响本书内容的Rails版本变更，可以访问Pragprog网站的官方wiki（http://www.pragprog.com/wikis/wiki/ChangesToRails）获取最新信息。作者Sam、Dave和David共同创作了这本书，他们在介绍中分享了他们对Rails框架的热情和对社区的理解。作者Mikel Lindsaar，作为Rails核心团队的成员和Ruby的创造者之一，表示该书不仅延续了早期版本的成功，而且在最新版中仍然保持着对新手开发者极高的价值，是他推荐给任何想要入门Rails的新手开发者的首选书籍。《敏捷Web开发与Rails 4th版》不仅涵盖了基础概念和技术，还注重与时俱进，引导读者适应Rails框架的演进，是任何希望在Web开发领域取得成功的人不可或缺的参考资料。"

Introduction

Ruby on Rails is a framework that makes it easier to develop, deploy, and

maintain web applications. During the months that followed its initial release,

Rails went from being an unknown toy to being a worldwide phenomenon, and

more important, it has become the framework of choice for the implementation

of a wide range of so-called W e b 2.0 applications.

Why is that?

Rails Simply F e e l s Right

First, a large number of developers were frustrated with the technologies they

were using to create web applications. It didn’t seem to matter whether they

were using Java, PHP, or .NET—there was a growing sense that their job was

just too damn hard. And then, suddenly, along came Rails, and Rails was

easier.

But easy on its own doesn’t cut it. W e ’ r e talking about professional developers

writing real-world websites. They wanted to feel that the applications they

were developing would stand the test of time—that they were designed and

implemented using modern, professional techniques. So, these developers dug

into Rails and discover ed it wasn’t just a tool for hacking out sites.

For example, all Rails applications are implemented using the Model-View-

Controller (MVC) architecture. Java developers are used to frameworks such

as Tapestryand Struts, which are based on MVC. But Rails takes MVC further:

when you develop in Rails, you start with a working application, there’s a

place for each piece of code, and all the pieces of your application interact in a

standard way.

Professional programmers write tests. And again, Rails delivers. All Rails appli-

cations have testing support baked right in. As you add functionality to the

code, Rails automatically creates test stubs for that functionality. The frame-

work makes it easy to test applications, and as a result, Rails applications

tend to get tested.

RAILSSIMPLYFEELSRIGHT 17

Rails applications are written in Ruby, a modern, object-oriented scripting

language. Ruby is concise without being unintelligibly terse—you can express

ideas naturally and cleanly in Ruby code. This leads to programs that are easy

to write and (just as important) are easy to read months later.

Rails takes Ruby to the limit, extending it in novel ways that make a pro-

grammer’s life easier. This makes our programs shorter and more readable.

It also allows us to perform tasks that would normally be done in external

conﬁguration ﬁles inside the codebase instead. This makes it far easier to see

what’s happening. The following code deﬁnes the model class for a project.

Don’t worry about the details for now. Instead, just think about how much

information is being expressed in a few lines of code.

class Project < ActiveRecord::Base

belongs_to :portfolio

has_one :project_manager

has_many :milestones

has_many :deliverables, :through => :milestones

validates :name, :description, :presence => true

validates :non_disclosure_agreement, :acceptance => true

validates :short_name, :uniqueness => true

end

Two other philosophical underpinnings keep Rails code short and readable:

DRY and convention over conﬁguration. DRY stands for don’t r e p e a t yourself :

every piece of knowledge in a system should be expressed in just one place.

Rails uses the power of Ruby to bring that to life. Y o u ’ l l ﬁnd very little duplica-

tion in a Rails application; you say what you need to say in one place—a place

often suggested by the conventions of the MVC architecture—and then move

on. For programmers used to other web frameworks, where a simple change

to the schema could involve them in half a dozen or more code changes, this

was a revelation.

Convention over conﬁguration is crucial, too. It means that Rails has sensi-

ble defaults for just about every aspect of knitting together your application.

Follow the conventions, and you can write a Rails application using less code

than a typical Java web application uses in XML conﬁguration. If you need to

override the conventions, Rails makes that easy, too.

Developers coming to Rails found something else, too. Rails isn’t playing catch-

up with the new de facto web standards; it’s helping deﬁne them. And Rails

makes it easy for developers to integrate features such as Ajax and RESTful

interfaces into their code, because support is built in. (And if you’re not famil-

iar with Ajax and REST interfaces, never fear—we’ll explain them later in the

book.)

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RAILSIS AGILE 18

Developers are worried about deployment too. They found that with Rails you

can deploy successive releases of your application to any number of servers

with a single command (and roll them back equally easily should the release

prove to be somewhat less than perfect).

Rails was extracted from a real-world, commercial application. It turns out

that the best way to create a framework is to ﬁnd the central themes in a

speciﬁc application and then bottle them up in a generic foundation of code.

When you’re developing your Rails application, you’re starting with half of a

really good application already in place.

But there’s something else to Rails—something that’s hard to describe. Some-

how, it just feels right. Of course, you’ll have to take our word for that until

you write some Rails applications for yourself (which should be in the next

forty-ﬁve minutes or so...). That’s what this book is all about.

Rails Is Agile

The title of this book is Agile W e b Development with Rails. Y o u may be sur-

prised to discover that we don’t have explicit sections on applying agile prac-

tices X, Y , and Z to Rails coding.

The reason is both simple and subtle. Agility is part of the fabric of Rails.

Let’s look at the values expressed in the Agile Manifesto as a set of four pref-

erences:

• Individuals and interactions over processes and tools

• W o r k i n g software over comprehensive documentation

• Customer collaboration over contract negotiation

• Responding to change over following a plan

Rails is all about individuals and interactions. There are no heavy toolsets,

no complex conﬁgurations, and no elaborate processes. There are just small

groups of developers, their favorite editors, and chunks of Ruby code. This

leads to transparency; what the developers do is reﬂected immediately in what

the customer sees. It’s an intrinsically interactive process.

Rails doesn’t denounce documentation. Rails makes it trivially easy to cre-

ate HTML documentation for your entire codebase. But the Rails development

process isn’t driven by documents. Y o u won’t ﬁnd 500-page speciﬁcations at

the heart of a Rails project. Instead, you’ll ﬁnd a group of users and devel-

opers jointly exploring their need and the possible ways of answering that

need. Y o u ’ l l ﬁnd solutions that change as both the developers and the users

become more experienced with the problems they’re trying to solve. Y o u ’ l l ﬁnd

1. http://agilemanifesto.org/. Dave Thomas was one of the seventeen authors of this document.

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WHOTHISBOOKIS FOR 19

a framework that delivers working software early in the development cycle.

This software may be rough around the edges, but it lets the users start to get

a glimpse of what you’ll be delivering.

In this way, Rails encourages customer collaboration. When customers see

just how quickly a Rails project can respond to change, they start to trust

that the team can deliver what’s required, not just what has been requested.

Confrontations are replaced by “What if?” sessions.

That’s all tied to the idea of being able to respond to change. The strong, almost

obsessive, way that Rails honors the DRY principle means that changes to

Rails applications impact a lot less code than the same changes would in other

frameworks. And since Rails applications are written in Ruby, where concepts

can be expressed accurately and concisely, changes tend to be localized and

easy to write. The deep emphasis on both unit and functional testing, along

with support for test ﬁxtures and stubs during testing, gives developers the

safety net they need when making those changes. W i t h a good set of tests in

place, changes are less nerve-racking.

Rather than constantly trying to tie Rails processes to the agile principles,

we’ve decided to let the framework speak for itself. As you read through the

tutorial chapters, try to imagine yourself developing web applications this way:

working alongside your customers and jointly determining priorities and solu-

tions to problems. Then, as you read the more advanced concepts that follow

in Part III, see how the underlying structure of Rails can enable you to meet

your customers’ needs faster and with less ceremony.

One last point about agility and Rails: although it’s probably unprofessional

to mention this, think how much fun the coding will be.

Who This Book Is For

This book is for programmers looking to build and deploy web-based applica-

tions. This includes application programmers who are new to Rails (and per-

haps even new to Ruby) and ones who are familiar with the basics but want a

more in-depth understanding of Rails.

W e presume some familiarity with HTML, Cascading Style Sheets (CSS), and

JavaScript, in other words, the ability to view source on web pages. Y o u do not

need to be an expert on these subjects; the most you will ever be expected to do

is to copy and paste material from the book, all of which can be downloaded.

How T o Read This Book

The ﬁrst part of this book makes sure you are ready. By the time you are done

with it, you will have been introduced to Ruby (the language), you will have

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HOWTO READTHISBOOK 20

been exposed to an overview of Rails itself, you will have both Ruby and Rails

installed, and you will have veriﬁed this installation with a simple example.

The next part takes you through the concepts behind Rails via an extended

example; we build a simple online store. It doesn’t take you one by one through

each component of Rails (“here is a chapter on models, here is a chapter on

views,” and so forth). These components are designed to work together, and

each chapter in this section tackles a speciﬁc set of related tasks that involve

a number of these components working together.

Most folks seem to enjoy building the application along with the book. If you

don’t want to do all that typing, you can cheat and download the source code

(a compressed tar ar chive or a zip ﬁle).

The third part of the book, starting on page 258, surveys the entire Rails

ecosystem. This starts with the functions and facilities of Rails that you will

now be familiar with. It then covers a number of key dependencies that the

Rails framework makes use of that contribute directly to the overall function-

ality that the Rails framework delivers. Finally, there is a survey of a number

of popular plugins that augment the Rails framework and make Rails an open

ecosystem rather than merely a framework.

Along the way, you’ll see various conventions we’ve adopted.

Live Code

Most of the code snippets we show come from full-length, running exam-

ples that you can download. To help you ﬁnd your way, if a code listing

can be found in the download, there’ll be a bar before the snippet (just

like the one here).

Download w o r k / d e m o 1 / a p p / c o n t r o l l e r s / s a y _ c o n t r o l l e r. r b

class SayController < ApplicationController

def hello

end

def goodbye

end

This contains the path to the code within the download. If you’re reading

the ebook version of this book and your ebook viewer supports hyper-

links, you can click the bar, and the code should appear in a br owser

window. Some browsers (such as Safari) will mistakenly try to interpret

some of the templates as HTML. If this happens, view the source of the

page to see the real source code.

2. http://pragprog.com/titles/rails4/source_code has the links for the downloads.

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Rails 4敏捷Web开发指南：保持同步与实战

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Agile Web Development with Rails 4th edition（敏捷Web开发与Rails:程序指南第四版）