Ruby 自检系列

本文基于 ruby-trivia 和 ruby-style完成，节选部分问题，并更进一步探究原理，用以加强自身对ruby基础的理解和应用。

Language Characteristics and Core Objects

Q: Is Ruby a statically typed or a dynamically typed language?
A: Dynamically typed since type checking is done at runtime.

Q: Is Ruby a strongly typed or a weakly typed language?
A: Strongly typed since an object's type is checked before an operation is performed on it.

Q: What is the difference between a statement and an expression in Ruby?
A: All statements are expressions in Ruby since all statements return a value.
(expressions > statements)

Data Types

Q: Does String include the Enumerable module?
A: No.

Q: What method might you use to enumerate over a string?
A: String#each_char

Hash

Q: How might you specify a default value for a hash?
A: Pass the default values as arguments to ::new on initialization or change the default directly with the method Hash#default. You may also provide a default at the time of query with Hash#fetch.
C: 在某些情况下使用Hash#fetch是比较合适的：

• 当处理应该存在的哈希键时，使用 Hash#fetch

heroes = { batman: 'Bruce Wayne', superman: 'Clark Kent' }

# 差 - 如果我们打错了哈希键，则难以发现这个错误
heroes[:batman] # => 'Bruce Wayne'
heroes[:supermann] # => nil

# 好 - fetch 会抛出 KeyError 使这个错误显而易见
heroes.fetch(:supermann)

• 当为哈希键的值提供默认值时，倾向使用 Hash#fetch 而不是自定义逻辑。

batman = { name: 'Bruce Wayne', is_evil: false }

# 差 - 如果仅仅使用 || 操作符，那么当值为假时，我们不会得到预期结果
batman[:is_evil] || true # => true

# 好 - fetch 在遇到假值时依然可以正确工作
batman.fetch(:is_evil, true) # => false

• 当提供默认值的求值代码具有副作用或开销较大时，倾向使用 Hash#fetch 的block形式

batman = { name: 'Bruce Wayne' }

# 差 - 此形式会立即求值，如果调用多次，可能会影响程序的性能
batman.fetch(:powers, obtain_batman_powers) # obtain_batman_powers 开销较大

# 好 - 此形式会惰性求值，只有抛出 KeyError 时，才会产生开销
batman.fetch(:powers) { obtain_batman_powers }

Q: Does Hash use #== or #eql? to compare hash keys?
A: #eql?
C: Why? What's the different between them？
Hash使用#eql?来做比较。而Numeric则是通过 #==做比较，而不是#eql？。在这种情况下，#eql？会做更严格的比较。
对于Object的对象, #eql? 是跟 #== 一样的。由此可知，Numeric是例外

equal? 则检查两者是否是同一对象（相同值，相同type）。

#对于 Numeric
x = 1
puts x.object_id  # => 3

y = 1
puts y.object_id  # => 3

z = 1.0
puts z.object_id  # => -36028797018963966

x == y  # true  同一个object
x.eql? y  # true 同一个object

x == z   #true 不同object，但是content相同
x.eql? z  #false 不同object，并且是不同的type，x是Fixnum，y是Float

#对于 Array object
x = [1, 2, 3]
y = [3, 2, 1]
z = [1, 2, 3]

x.object_id  # => 70244601475800
y.object_id  # => 70244601492000
x.eql?(y) #=> false

x.object_id  # => 70244601475800
z.object_id  # => 70244601475800
x.eql?(z) #=> true

x.object_id  # => 70244601475800
s = y.sort  # => [1, 2, 3]
s.object_id  # => 70244618664060
x.class # => Array
y.class # => Array
x == s #=> true  相同value
x.eql?(s) #=> true  相同value和type
x.equal?s  #=> false 相同value和type，但是不是同一个对象

Q: What's is the precedence of &&, and, or, =, ||?
A: && > || > = > (and, or)

Q: . vs ::
A: 调用类方法时，他们没有区别。但是使用::可以访问constant和namespace

Q: != vs <=>(spaceship)
A: != 不等于。 <=>，左边大于右边则返回1，等于返回0，小于返回-1

Q: What's true and false in ruby?
A: Everything in Ruby is true except false and nil.
TrueClass, FalseClass, but can't create a new instance for them. true and false are the only things we can get.
We can recognize them by either using nil? or false on == left-hand.

Q: What is duck type?
A: Walk like a duck, yip like a duck, it's a duck. In an other word, interface over type.

Q:

h = Hash.new
h[:s] = 'ok'

Can you retrive 'ok' by h['s'] ?
A: You cannot. Since the Hash class in Ruby’s core library retrieves values by doing a standard == comparison on the keys. This means that a value stored for a Symbol key (e.g. :my_value) cannot be retrieved using the equivalent String (e.g. ‘my_value’). On the other hand, HashWithIndifferentAccess treats Symbol keys and String keys as equivalent so that the following would work:

Q:#joins vs #includes in Rails ActiveRecord
A: #joins performs an inner join between two tables.

orders = Order.joins(:listing)
=> SELECT "orders".* FROM "orders" INNER JOIN "listings" ON "listings"."id" = "orders"."listing_id"

内连接。对于 T1 中的每一行 R1 ，如果能在 T2 中找到一个或多个满足连接条件的行， 那么这些满足条件的每一行都在连接表中生成一行。In another word, It will retrieve all records where listing_id (of orders table) is equal to listing.id (listings table)

这时候并没有listing的数据被取出。所有如果要取某个order的关联关系listing的属性，还是回去查一次数据库：

order_1 = orders.first
//上个命令的执行结果被存在内存中了, 直接读取了。
=> #<Order id: 1, customer_id: 1, listing_id: 1, product_id: 1, created_at: "2016-04-08 02:30:16", updated_at: "2016-04-08 02:30:49", status: "paid">

order_1.listing.address
//但具体关联到某个order上的listing信息还在数据库中，需要进行一次查询。
=> SELECT  "listings".* FROM "listings" WHERE "listings"."id" = $1 LIMIT 1  [["id", 1]]

#includes performs a left outer like join between the two tables.

orders = Order.includes(:listing)
=> SELECT "orders".* FROM "orders"
=> SELECT "listings".* FROM "listings" WHERE "listings"."id" IN (1, 2, 3, 4)

这时候执行的是类似于左外连接。
左外连接：首先执行一次内连接。然后为每一个 T1 中无法在 T2 中找到匹配的行生成一行， 该行中对应 T2 的列用 NULL 补齐。因此，生成的连接表里总是包含来自 T1 里的每一行至少一个副本。
为什么说是‘类似左外连接’？正常的左外连接应该是这样写的：

SELECT "orders".* FROM "orders" LEFT JOIN "listings" ON "listings"."id" = "orders"."listing_id";

但是在rails中我们并没有看到。这是因为在rails4中，我们需要手动写LEFT OUTER JOIN的sql查询语句：

orders = Order.joins('LEFT OUTER JOIN "listings" ON "listings"."id" = "orders"."listing_id"')

其实rails帮我们做的是先把所有orders表里的记录取出。然后再在listings表中取出可以跟orders表记录匹配的记录，并存在内存中。

好消息是rails5中加入了一个新方法#letf_outer_joins来执行LEFT OUTER JOINS.

这时候我们再来执行一次查询某个order的关联关系listings上的属性：

order_1 = orders.first
//从内存中取
=> #<Order id: 1, customer_id: 1, listing_id: 1, product_id: 1, created_at: "2016-04-08 02:30:16", updated_at: "2016-04-08 02:30:49", status: "paid">

order_1.listing.address
//因为listings表中的相应记录已经在第一步中被取出，所以这时候直接从内存中读即可。
 => {"city"=>"Nantes (44000)", "street_number"=>"", "region"=>"Pays de la Loire", "street"=>""}

可见，再次查询的时候并不会去读数据库。