Sometimes, controllers just suck. Time to extract.

my_controller.rb

class MyController < ApplicationController
  def create
    @model = Model.new(model_params)
    
    # we have to call this here, to populate
    # errors on any other fields that might
    # be invalid
    @model.valid?

    if @model.model_attr == 'invalid' && [email protected]_state(:dont_care)
      @model.errors.add(:model_attr, :invalid)
    end

    if @model.other_attr == 'what is this i am not good with computer' &&
       !controller_derived_list.include?(@model.yet_anoth_attr) &&
       !plz_stop_conditionals
      
      @models.errors.add(:oh_god, :why)
    elsif @conditional
      redirect_to other_place, error: "noooooo #{@model.errors.full_messages.join}"
    end

    # have to use this, since #valid? clears errors
    if @model.errors.any?
      render 'new' and return
    end

    elsif @model.save
      if params[:notice_one]
        flash[:notice] = "2 many conditionals"
      else
        flash[:notice] = "fail"
      end
      
      redirect_to next_path, notice: "you have no idea how many if statements just went right"
    else
      render 'new'
    end
  end

  private

  def model_params
    params.require(:model).permit(:attr1, :attr2, 
      submodel: [:name, {this_doesnt: :work}],
      associated_ids: [] # wat
    )
  end
end