Cannot create an new interface when it belongs to project - ruby-on-rails

Question

I try to create a new interface object. After clicking create button, it still remains new.html.erb, it should go to project_interfaces_path(main page). Also, the data has not saved yet.
I have tried many ways such as change URL, but it does not work and it reports NoMethodError in InterfacesController#create
undefined method `interfaces' for nil:NilClass
The interface/new.html.erb
<div class="card-body">
<%= form_for #interface, url:project_interfaces_path,method: :post do |f| %>
<div class="form-group">
<%= f.label :name %>
<%= f.text_area :name,class: 'form-control'%>
</div>
<div class="form-group">
<%= f.label :desp %>
<%= f.text_field :desp,class:'form-control'%>
</div>
<div class="form-group">
<%= f.label :request_url %>
<%= f.text_field :request_url,class:'form-control'%>
</div>
<div class="form-group">
<%= f.label :request_eg %>
<%= f.text_field :request_eg,class:'form-control'%>
</div>
<div class="form-group">
<%= f.label :response_eg %>
<%= f.text_field :response_eg,class:'form-control'%>
</div>
<%=link_to project_interfaces_path do%>
<button type="button" class="btn btn-primary">返回列表</button>
<% end %>
<%=f.submit "创建",class: 'btn btn-primary' %>
<% end %>
The interface controller：
def new
#interface = Interface.new
end
def create
#interface = #project.interfaces.new(interface_params)
if #interface.save
redirect_to project_interfaces_path
else
render :new
end
end
private
def interface_params
params.require(:interface).permit(:id, :name, :desp,:request_url,:request_eg,:response_eg)
end
The interface belongs to project:
class Interface < ApplicationRecord
belongs_to :method_type
has_many :get_fields, dependent: :destroy
has_many :put_fields, dependent: :destroy
belongs_to :project
end

Answer 1

You're working with nested resources, it means you can't create an interface without project_id, since Interface belongs_to :project. How it should be:
def new
#project = Project.find(params[:project_id])
#interface = #project.interfaces.new
end
def create
#project = Project.find(params[:project_id])
#interface = #project.interfaces.build(interface_params)
if #interface.save
redirect_to project_interfaces_path(#project)
else
render :new
end
end
private
def interface_params
params.require(:interface).permit(:id, :name, :desp,:request_url,:request_eg,:response_eg)
end
And remove url and method options from form, it works automagically
<%= form_for #interface do |f| %>

Answer 2

Indeed, you are redirecting to new instead of project_interfaces_path:
def create
#interface = Interface.new(interface_params)
if #interface.save
#redirect_to new_project_interface_path(project) <- wrong path
redirect_to project_interfaces_path # Good path
else
render :new
end
end
Also, add a space between url: and project_interfaces_path in <%= form_for #interface, url:project_interfaces_path,method: :post do |f| %>.
UPDATE: It seems you are trying to save an Interface without associate a Project to it.
You need to retrieve a project and build the interface with it:
def new
project = Project.find(params[:id]) # Assuming you are sending it
#interface = project.interfaces.build
end
def create
project = Project.find(params[:id]) # Assuming you are sending it
#interface = project.interfaces.build(interface_params)
if #interface.save
redirect_to project_interfaces_path
else
render :new
end
end
Taking a look on your routes would help.