ConcurrencyMiddleware

You can globally intercept RecordModifiedError adding ConcurrencyMiddleware to your MIDDLEWARE_CLASSES. Each time a RecordModifiedError is raised it goes up to the ConcurrencyMiddleware and the handler defined in CONCURRENCY_HANDLER409 is invoked.

Example

settings.py

MIDDLEWARE_CLASSES=('django.middleware.common.CommonMiddleware',
                    'concurrency.middleware.ConcurrencyMiddleware',
                    'django.contrib.sessions.middleware.SessionMiddleware',
                    'django.middleware.csrf.CsrfViewMiddleware',
                    'django.contrib.auth.middleware.AuthenticationMiddleware',
                    'django.contrib.messages.middleware.MessageMiddleware')

CONCURRENCY_HANDLER409 = 'demoproject.demoapp.views.conflict'
CONCURRENCY_POLICY = 2  # CONCURRENCY_LIST_EDITABLE_POLICY_ABORT_ALL

views.py

from diff_match_patch import diff_match_patch
from concurrency.views import ConflictResponse
from django.template import loader
from django.utils.safestring import mark_safe
from django.template.context import RequestContext

def get_diff(current, stored):
    data = []
    dmp = diff_match_patch()
    fields = current._meta.fields
    for field in fields:
        v1 = getattr(current, field.name, "")
        v2 = getattr(stored, field.name, "")
        diff = dmp.diff_main(unicode(v1), unicode(v2))
        dmp.diff_cleanupSemantic(diff)
        html = dmp.diff_prettyHtml(diff)
        html = mark_safe(html)
        data.append((field, v1, v2, html))
    return data

def conflict(request, target=None, template_name='409.html'):
    template = loader.get_template(template_name)
    try:
        saved = target.__class__._default_manager.get(pk=target.pk)
        diff = get_diff(target, saved)
    except target.__class__.DoesNotExists:
        saved = None
        diff = None

    ctx = RequestContext(request, {'target': target,
                                   'diff': diff,
                                   'saved': saved,
                                   'request_path': request.path})
    return ConflictResponse(template.render(ctx))

409.html

{% load concurrency %}
<table>
    <tr>
        <th>
            Field
        </th>
        <th>
            Current
        </th>
        <th>
            Stored
        </th>
        <th>
            Diff
        </th>

    </tr>
    <tr>
        {% for field, current, stored, entry in diff %}
            {% if not field.primary_key and not field|is_version %}
                <tr>
                    <td>
                        {{ field.verbose_name }}
                    </td>
                    <td>
                        {{ current }}
                    </td>
                    <td>
                        {{ stored }}
                    </td>
                    <td>
                        {{ entry }}
                    </td>
                </tr>
            {% endif %}
        {% endfor %}
    </tr>
</table>

If you want to use ConcurrencyMiddleware in the admin and you are using concurrency.admin.ConcurrentModelAdmin remember to set your ModelAdmin to NOT use concurrency.forms.ConcurrentForm

from django import forms

class MyModelAdmin(ConcurrentModelAdmin):
    form = forms.ModelForm  # overrides default ConcurrentForm