Django Pagination object of type 'NoneType' has no len()

Question

view.py

def search(request):
    query_string = ''
    found_entries = None
    if ('q' in request.GET) and request.GET['q'].strip():
        query_string = request.GET['q']
        entry_query = get_query(query_string, ['id', 'sample_name'])
        found_entries = Sample.objects.filter(entry_query).exclude(status_id=3).order_by('id')

    paginator = Paginator(found_entries, 30)
    page = request.GET.get('page')
    try:
        sample = paginator.page(page)
    except PageNotAnInteger:
        sample = paginator.page(1)
    except EmptyPage:
        sample = paginator.page(paginator.num_pages)
    return render_to_response('search_results.html', { 'sample' : sample })

search_result.html

{% extends 'base.html' %}
{% block content %}
<table class="reference" style="width:100%">
<tr>
    <th>Tracking ID</th>
    <th>Sample Name</th>
    <th>External ID</th>
    <th>Owner Name</th>
    <th>Custodian Name</th>
    <th>Feedstock</th>
    <th>Treatment</th>
    <th>Fraction</th>
    <th>Create Date</th>
    <th>Update</th>
    <th>Delete</th>
</tr>

{% for result in sample %}
<tr>
    <td>{{result.id}}</td>
    <td>{{result.sample_name}}</td>
    <td>{{result.external_id}}</td>
    <td>{{result.owner_name}}</td>
    <td>{{result.custodian_name}}</td>
    <td>{{result.feedstock.feedstock_name}}</td>
    <td>{{result.treatment.treatment_name}}</td>
    <td>{{result.fraction.fraction_name}}</td>
    <td>{{result.create_date}}</td>
    <td><a href="/sdms/update/{{result.id}}">update</a></td>
    <td><a href="/sdms/delete/{{result.id}}">delete</a></td>
</tr>

{% endfor %}
<div class="pagination">
<span class="step-links">
    {% if sample.has_previous %}
        <a href="?page={{ sample.previous_page_number }}">previous</a>
    {% endif %}

    <span class="current">
        Page {{ sample.number }} of {{ sample.paginator.num_pages }}
    </span>

    {% if sample.has_next %}
        <a href="?page={{ sample.next_page_number }}">next</a>
    {% endif %}
</span>
</div>

The initial page is displayed correctly. It has 30 items displayed in a table. It also display 1 of 10 pages with the link for the next page. When I click the link I get an error: object of type 'NoneType' has no len()

Here is the traceback:

File "c:\python27\lib\site-packages\django-1.7.1-py2.7.egg\django\core\handlers\base.py" in get_response
111. response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "C:\Users\vtran.NREL_NT\PycharmProjects\sdms\sdms\sample\views.py" in search
66. sample = paginator.page(page)
File "c:\python27\lib\site-packages\django-1.7.1-py2.7.egg\django\core\paginator.py" in page
50. number = self.validate_number(number)
File "c:\python27\lib\site-packages\django-1.7.1-py2.7.egg\django\core\paginator.py" in validate_number
39. if number > self.num_pages:
File "c:\python27\lib\site-packages\django-1.7.1-py2.7.egg\django\core\paginator.py" in _get_num_pages
86. if self.count == 0 and not self.allow_empty_first_page:
File "c:\python27\lib\site-packages\django-1.7.1-py2.7.egg\django\core\paginator.py" in _get_count
77. self._count = len(self.object_list)

Answer 1

您的next链接不包含查询字符串，因此当您单击它时，生成的下一个请求将离开found_entries = None ，然后您将其传递到期望项目列表的Paginator 。

Answer 2

You've set your link to be ?page=whatever . So you're only sending the page number in the link. But your view checks for the presence of a q parameter, and only if it's there does it override the value of found_entries .

You'll need to preserve the q parameter in the next/previous page links.