Chants: pagination: preserve filter parameters

Author: igneus

scores/templates/scores/_pagination.html

@@ -1,14 +1,16 @@
+{% load params %}
+
 {% if page_obj %}
 <nav aria-label="Page navigation" class="mt-4">
   <ul class="pagination justify-content-center">
     {% if page_obj.has_previous %}
       <li class="page-item">
-        <a class="page-link" href="?page=1" aria-label="First">
+        <a class="page-link" href="?{% query_transform page=1 %}" aria-label="First">
           <span aria-hidden="true">&laquo;&laquo;</span>
         </a>
       </li>
       <li class="page-item">
-        <a class="page-link" href="?page={{ page_obj.previous_page_number }}" aria-label="Previous">
+        <a class="page-link" href="?{% query_transform page=page_obj.previous_page_number %}" aria-label="Previous">
           <span aria-hidden="true">&laquo;</span>
         </a>
       </li>
@@ -21,19 +23,19 @@
         </li>
       {% elif num > page_obj.number|add:'-3' and num < page_obj.number|add:'3' %}
         <li class="page-item">
-          <a class="page-link" href="?page={{ num }}">{{ num }}</a>
+          <a class="page-link" href="?{% query_transform page=num %}">{{ num }}</a>
         </li>
       {% endif %}
     {% endfor %}
 
     {% if page_obj.has_next %}
       <li class="page-item">
-        <a class="page-link" href="?page={{ page_obj.next_page_number }}" aria-label="Next">
+        <a class="page-link" href="?{% query_transform page=page_obj.next_page_number %}" aria-label="Next">
           <span aria-hidden="true">&raquo;</span>
         </a>
       </li>
       <li class="page-item">
-        <a class="page-link" href="?page={{ page_obj.paginator.num_pages }}" aria-label="Last">
+        <a class="page-link" href="?{% query_transform page=page_obj.paginator.num_pages %}" aria-label="Last">
           <span aria-hidden="true">&raquo;&raquo;</span>
         </a>
       </li>

scores/templatetags/__init__.py

@@ -0,0 +1,23 @@
+# credits: https://gist.github.com/benbacardi/d6cd0fb8c85e1547c3c60f95f5b2d5e1
+from django import template
+
+register = template.Library()
+
+
+@register.simple_tag(takes_context=True)
+def query_transform(context, **kwargs):
+    '''
+    Returns the URL-encoded querystring for the current page,
+    updating the params with the key/value pairs passed to the tag.
+
+    E.g: given the querystring ?foo=1&bar=2
+    {% query_transform bar=3 %} outputs ?foo=1&bar=3
+    {% query_transform foo='baz' %} outputs ?foo=baz&bar=2
+    {% query_transform foo='one' bar='two' baz=99 %} outputs ?foo=one&bar=two&baz=99
+
+    A RequestContext is required for access to the current querystring.
+    '''
+    query = context['request'].GET.copy()
+    for k, v in kwargs.items():
+        query[k] = v
+    return query.urlencode()