Note: The information provided here is intended for beginners.

While attempting to change the image in a link_to element upon hovering over it, I encountered difficulty achieving the desired effect using CSS. Below, I share the solution I implemented to resolve this issue.

This is the link_to code snippet:

  <%= link_to image_tag('1487789814_back.png'),
      {controller: @controller_action_hash_from_params[:controller],
        action: @controller_action_hash_from_params[:action]},
        sql_text: @sql_text,
      :id => "ralph-back-button-black-img", :class => "ralph-fieldset-back-button" %>

The image '1487789814_back.png' is a simple PNG file located in app/assets/images, depicting a black arrow pointing left. Additionally, '1487789814_back_red.png' from the same directory features a red arrow pointing left. My goal was to have the red arrow display when hovering over a fieldset containing the black arrow.

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Identifying the Issue:

The HTML generated by the aforementioned link_to code snippet appeared as follows:

<img src="https://localhost:3000/assets/1487789814_back-731afaae70e04062b25988079e4ef8ab4e6b523f4ac11ff7d142a9ff01e63455.png" alt="1487789814 back">

Please note that the URL 'https://localhost:3000/assets/1487789814_back-731afa...3455.png' may not be accessible as it is an automatically assigned filename by Rails.

Analysis and Resolution Approach:

The issue partly arises because, according to my understanding, CSS cannot override certain HTML attributes such as the 'src' attribute of an image. Hence, conventional CSS techniques like using ':hover' would not work in this scenario. To address this challenge, resorting to JavaScript or jQuery was necessary.

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Solution Implemented:

I wish to clarify that while the solution I present worked within my Rails 4 environment, I do not claim it to be the definitive or optimal answer.

If you are utilizing the asset pipeline feature in your Rails project (applicable to versions including Rails 3 and Rails 5), the user-friendly image filenames like '1487789814_back.png' undergo conversion by the asset pipeline system before deployment, resulting in hash-appended filenames. Thus, gaining access to the red arrow image requires knowledge of its corresponding filename within the asset pipeline.

To establish this mapping between the friendly name and asset pipeline name, I leveraged a hidden div styled with 'display:none'.

Here's how it was done in ERB:

<%# This div has display:none.
    We do this in order to preload the images_path as well as create a "map"
      between the friendly name and the asset pipeline name
%>
<div class='ralph_preload'>
  <%= image_tag('1487789814_back.png',      options={class:'ralph_preload_class', id:'1487789814_back'}) %>
  <%= image_tag('1487789814_back_red.png',  options={class:'ralph_preload_class', id:'1487789814_back_red'}) %>
</div>

The resulting HTML produced by the above ERB segment is as follows:

<div class="ralph_preload">
  <img class="ralph_preload_class" id="1487789814_back" src="/assets/1487789814_back-731afaae70e04062b25988079e4ef8ab4e6b523f4ac11ff7d142a9ff01e63455.png" alt="1487789814 back">
  <img class="ralph_preload_class" id="1487789814_back_red" src="/assets/1487789814_back_red-bbd0f2e34f3401cc46d2e4e1e853d472d8c01c9a75f96d78032738bd01b2133b.png" alt="1487789814 back red">
</div>

The associated CSS for this setup is defined as:

.ralph_preload {
  display: none;
}

Furthermore, jQuery scripting was incorporated to facilitate switching the black arrow image to the red variant upon hover:

<script type="text/javascript">
  function get_precache_myStruct_array(outer_div_class){
    var myStruct_array = [];
    $('.'+outer_div_class).find(".ralph_preload_class").each(function(){
      var myStruct = {
        id: this.id,
        src: this.src
      };
      // myStruct_array.push(myStruct);
      myStruct_array[this.id] = myStruct;
    });
    return myStruct_array;
  }

  function is_cached(img_url){
    var imgEle = document.createElement("img");
    imgEle.src = img_url;
    return imgEle.complete || (imgEle.width+imgEle.height) > 0;
  }

  $(document).ready(function(){
    var precache_myStruct_array = get_precache_myStruct_array("ralph_preload");
    $.each(precache_myStruct_array,
      function (index, value) {
        if (!is_cached(value.src)) {
          alert("Not cached!: " + value.src);
        };
    });

    <% if true %>
      $('#ralph-back-button-filedset').hover(function(){
        var precache_myStruct_array = get_precache_myStruct_array("ralph_preload");
        var imageID   = '1487789814_back_red';
        $('#ralph-back-button-black-img > img:nth-child(1)').attr("src", precache_myStruct_array[imageID].src);
      }, function(){
        var precache_myStruct_array = get_precache_myStruct_array("ralph_preload");
        var imageID = '1487789814_back';
        $('#ralph-back-button-black-img > img:nth-child(1)').attr("src", precache_myStruct_array[imageID].src);
      });
    <% end %>

  });
</script>

Within the jQuery code block, the "<% if true %>" / "<% end %>" section serves an inline processing purpose during ERB evaluation. Modifying this to false can aid in debugging using browser tools.

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Final Remarks:

In my quest to find a solution to the specified problem, I scoured online resources but didn't come across a direct match to my requirements. I welcome any feedback or corrections to improve the approach discussed herein.