_form.html.erb

<%= simple_form_for [@user, @message] do |f| %>
  <%= f.input :recipient %>
  <%= f.input :body %>
  
  <%= f.button :submit, class: "new btn-primary" %>
<% end %>

index.html.erb

<div class='page-header'>
  <h1>New Message: </h1>
</div>

<%= render 'form' %>
<br>

<h3>Messages: </h3>
<ul>
  <% @user.messages.each do |message| %>
    This message was sent <%= time_ago_in_words(message.created_at) %> ago
    from <%= message.author.email %>
    <li><%= message.body %></li>
  <% end %>
<ul>

message.rb

class Message < ActiveRecord::Base
  belongs_to :author, class_name: "User"
  validates :body, presence: true
  validates :recipient, presence: true
end

Messages Controller

class MessagesController < ApplicationController
  before_action :set_user
  
  def index
    @messages = @user.messages
    @message = Message.new
  end
  
  def create
    @message = Message.create(message_params)
    @message.author = current_user
    @message.recipient = User.find(params[:user_id])
    
    if @message.save
      flash[:notice] = "Message has been sent."
      redirect_to user_messages_path(current_user)
    else
      flash.now[:alert] = "Message has not been sent."
      render 'form'
    end
  end
  
  private
  
  def set_user
    @user = User.find(params[:user_id])
  end
  
  def message_params
    params.require(:message).permit(:body, :recipient)
  end
end

User.rb

class User < ActiveRecord::Base
  has_many :comments, foreign_key: "author_id"
  has_many :messages, foreign_key: "author_id"
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable
end