构建一个对象（或多个对象）并返回构建好的对象或对象的列表。

attributes 参数可以是 Hash 或 Array 的 Hashes。这些 Hashes 描述了要构建的对象上的属性。

示例¶ ↑

# Build a single new object
User.build(first_name: 'Jamie')

# Build an Array of new objects
User.build([{ first_name: 'Jamie' }, { first_name: 'Jeremy' }])

# Build a single object and pass it into a block to set other attributes.
User.build(first_name: 'Jamie') do |u|
  u.is_admin = false
end

# Building an Array of new objects using a block, where the block is executed for each object:
User.build([{ first_name: 'Jamie' }, { first_name: 'Jeremy' }]) do |u|
  u.is_admin = false
end

创建对象（或多个对象）并将其保存到数据库，前提是验证通过。无论对象是否成功保存到数据库，都会返回结果对象。

attributes 参数可以是 Hash 或 Array 的 Hashes。这些 Hashes 描述了要创建的对象上的属性。

示例¶ ↑

# Create a single new object
User.create(first_name: 'Jamie')

# Create an Array of new objects
User.create([{ first_name: 'Jamie' }, { first_name: 'Jeremy' }])

# Create a single object and pass it into a block to set other attributes.
User.create(first_name: 'Jamie') do |u|
  u.is_admin = false
end

# Creating an Array of new objects using a block, where the block is executed for each object:
User.create([{ first_name: 'Jamie' }, { first_name: 'Jeremy' }]) do |u|
  u.is_admin = false
end

创建对象（或多个对象）并将其保存到数据库，前提是验证通过。如果验证失败，将引发 RecordInvalid 错误，这与 Base#create 不同。

attributes 参数可以是 Hash 或 Array 的 Hashes。这些描述了要在对象上创建的属性，或者在给出 Array 的 Hashes 时创建多个对象。

给定一个属性哈希，instantiate 会返回一个适当类的最新实例。只接受字符串作为键。

例如，Post.all 可能会通过在 type 属性中存储记录的子类来返回 Comments、Messages 和 Emails。通过调用 instantiate 而不是 new，查找方法可以确保为每个记录获取相应类的最新实例。

请参阅 ActiveRecord::Inheritance#discriminate_class_for_record 以了解此“单表”继承映射的实现方式。

接受一个属性名列表，用于 SELECT / UPDATE / DELETE 查询的 WHERE 子句以及 first 和 last 查找方法的 ORDER BY 子句。

class Developer < ActiveRecord::Base
  query_constraints :company_id, :id
end

developer = Developer.first
# SELECT "developers".* FROM "developers" ORDER BY "developers"."company_id" ASC, "developers"."id" ASC LIMIT 1
developer.inspect # => #<Developer id: 1, company_id: 1, ...>

developer.update!(name: "Nikita")
# UPDATE "developers" SET "name" = 'Nikita' WHERE "developers"."company_id" = 1 AND "developers"."id" = 1

# It is possible to update an attribute used in the query_constraints clause:
developer.update!(company_id: 2)
# UPDATE "developers" SET "company_id" = 2 WHERE "developers"."company_id" = 1 AND "developers"."id" = 1

developer.name = "Bob"
developer.save!
# UPDATE "developers" SET "name" = 'Bob' WHERE "developers"."company_id" = 1 AND "developers"."id" = 1

developer.destroy!
# DELETE FROM "developers" WHERE "developers"."company_id" = 1 AND "developers"."id" = 1

developer.delete
# DELETE FROM "developers" WHERE "developers"."company_id" = 1 AND "developers"."id" = 1

developer.reload
# SELECT "developers".* FROM "developers" WHERE "developers"."company_id" = 1 AND "developers"."id" = 1 LIMIT 1

更新对象（或多个对象）并将其保存到数据库，前提是验证通过。无论对象是否成功保存到数据库，都会返回结果对象。

参数¶ ↑

• id - 这应该是要更新的 id 或 id 数组。可选参数，默认为关系中的所有记录。

• attributes - 这应该是一个属性哈希或哈希数组。

示例¶ ↑

# Updates one record
Person.update(15, user_name: "Samuel", group: "expert")

# Updates multiple records
people = { 1 => { "first_name" => "David" }, 2 => { "first_name" => "Jeremy" } }
Person.update(people.keys, people.values)

# Updates multiple records from the result of a relation
people = Person.where(group: "expert")
people.update(group: "masters")

注意：更新大量记录将为每条记录运行一个 UPDATE 查询，这可能会导致性能问题。当不需要为每条记录更新运行回调时，最好使用 update_all 来在单个查询中更新所有记录。

像 update 一样更新对象（或多个对象），但调用 update! 而不是 update，因此如果记录无效且保存失败，将引发异常。