为什么对未赋值变量没有错误？

Question

我有一个小学生模型和一个小组模型。当添加一个新的瞳孔时，我有一个collection_select框，其中有:multiple=> true，这样就可以将瞳孔分成几组。

<div class="field">
    <%= f.label "All Groups" %><br />
    <%= collection_select(:groups, :id, @all_groups,
                          :id, :name, {}, 
                          {:multiple => true}) %>
</div>

我有一个编辑瞳孔形式，当加载时，选择以前分配给瞳孔的组，以便在需要时可以更改它们，在集合选择选项中具有{}的额外位；

  <div class="field">
    <%= f.label "All Groups" %><br />
    <%= collection_select(:groups, :id, @all_groups,
                          :id, :name, {selected: @previous_selection},
                          {:multiple => true}) %>
</div>

@previous_selection设置在pupils_controller中；

  @previous_selection = Array.new
    @pupil.groups.each do |pg|
    @previous_selection.push(pg.id)
end

这是在def编辑块中，因此只为编辑页设置。

这里是PupilsController；

class PupilsController < ApplicationController
before_action :set_pupil, only: [:show, :edit, :update, :destroy]

def index
@pupils = Pupil.all
end

def show
  @pupil_groups = @pupil.groups
end


def new
@pupil = Pupil.new
@all_groups = set_pupil_list
end

def edit
  @all_groups = set_pupil_list

  @previous_selection = Array.new
   @pupil.groups.each do |pg|
  @previous_selection.push(pg.id)
end
end


def create
@pupil = Pupil.new(pupil_params)
clean_select_multiple_params
logger.debug "The groups parameter contains: #{params[:groups][:id]}"
selected_groups = Group.find(params[:groups][:id])
@pupil.groups = selected_groups

respond_to do |format|
  if @pupil.save
    format.html { redirect_to @pupil, notice: 'Pupil was successfully created.' }
    format.json { render action: 'show', status: :created, location: @pupil }
  else
    format.html { render action: 'new' }
    format.json { render json: @pupil.errors, status: :unprocessable_entity }
  end
end
end

def update
respond_to do |format|
  if @pupil.update(pupil_params)
    clean_select_multiple_params
    selected_groups = Group.find(params[:groups][:id])
    @pupil.groups = selected_groups
    format.html { redirect_to @pupil, notice: 'Pupil was successfully updated.' }
    format.json { head :no_content }
  else
    format.html { render action: 'edit' }
    format.json { render json: @pupil.errors, status: :unprocessable_entity }
  end
end
end

def destroy
@pupil.destroy
respond_to do |format|
  format.html { redirect_to pupils_url }
  format.json { head :no_content }
end
end

def full_name
@fn = @pupil.given_name
@sn = @pupil.surname
@full_name = @fn + @sn
end

private

def set_pupil
  @pupil = Pupil.find(params[:id])
end

def set_pupil_list
Group.all
end


def clean_select_multiple_params hash = params
hash.each do |k, v|
  case v
  when Array then v.reject!(&:blank?)
  when Hash then clean_select_multiple_params(v)
end
end
end

def pupil_params
  params.require(:pupil).permit(:given_name, :surname, :date_of_birth, :gender, :ethnicity)
end
end

当请求新的瞳孔页时，将使用_form.html.erb文件，该文件中包含{selected：@previous_selection}参数，而def在pupils_controller中尚未设置该参数，但没有关于未初始化@previous_selection的错误消息。

我期待一个错误，但没有得到一个，但不明白为什么。谁能解释一下吗？我对编程很陌生，所以如果我使用了错误的终端，很抱歉。

谢谢

里昂

Answer 1

@previous_selection变量是nil，因此在视图中不会选择任何collection项。没有必要将variable初始化为nil，rails会这样做。