Ruby-on-rails 如何仅从模型中选择特定属性？
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How to select only specific attributes from a model?
提问by Shadwell
I want to select only specific attributes from a model(id,name).
我只想从模型（id，name）中选择特定的属性。
The SQL-command can be for example:
例如，SQL 命令可以是：
SELECT id,name,username FROM Users
SELECT id,name,username FROM Users
Do you know how I can handle this?
你知道我该如何处理吗？
回答by Ross
Pretty old question, but the rails 3 way would be:
很老的问题，但 Rails 3 的方式是：
User.pluck(:id)
回答by Shadwell
There's a :selectoption on find methods. This allows you to do:
:selectfind 方法有一个选项。这允许您执行以下操作：
User.find(:all, :select => 'id, name, username')
The returned objects will be Userinstances with those attributes available.
返回的对象将是User具有可用属性的实例。
Or if you really want just the values without wrapping them us as Userinstances. You can add a method to Userto return them.
或者，如果您真的只想要这些值而不将它们包装为我们的User实例。您可以添加一个方法User来返回它们。
def self.get_ids_and_names
self.connection.select_all("select id, name, username from users")
end
which will return an array of hashes mapping column name to the value for that row. E.g. [{'id' => 1, 'name' => 'user1', 'username' => 'username1'}, ... ]
这将返回一个哈希数组，将列名映射到该行的值。例如[{'id' => 1, 'name' => 'user1', 'username' => 'username1'}, ... ]
回答by ez.
You can also do
你也可以这样做
User.find(:all).map(&:id)
to get a list of the user ids if you are trying to get a list of user's ids or names
如果您尝试获取用户 ID 或名称列表，则获取用户 ID 列表
回答by Xavier
You can also pass in an array, like so:
您还可以传入一个数组，如下所示：
Model.all.select ['id', 'title']
回答by Khanh Pham
We can use selectfor symbolor stringsuch as:
我们可以select用于symbol或string例如：
User.select("id, name, username")
or
或者
User.select(:id, :name, :username)
回答by Ezequiel García
actually for you only need write this
其实你只需要写这个
@user= User.select(:attributeN, :attributeN......., attributeN)
respond_to do |format|
format.json {
render json: @user
}
回答by Daniel Szmulewicz
In mongoid, that would be:
在 mongoid 中，这将是：
User.only(:name, :username).where(id: params[:id]).first
回答by imechemi
SQL
SQL
SELECT id,name,username FROM Users
SELECT id,name,username FROM Users
RUBY
红宝石
User.select(:id, :name, :username)
User.select(:id, :name, :username)
with condition, you can write like:
User.select(:id, :name, :username).find_by(:age 22)
有条件，你可以这样写：
User.select(:id, :name, :username).find_by(:age 22)
