如何在Django中合并两个查询并选择与两个不相关的表不同

Question

I'm working in Django (1.8) with a MySQL backend. I have two unrelated tables that I want to query. The results are a concatenation of two columns from each table. I want to merge the results, sort them and select distinct. For the sake of example let's assume I have tableA and tableB both of which have first_name and last_name columns. Each cloumn heading has a reference to the table in it (eg first_name_A). Currently I'm running a query on each table and using sorted(chain(queryA, queryB)) to put them together. The problem comes when I try to reference the result in a for loop in the template.

Codewise I have this:

views.py

queryA = tableA.objects.order_by('first_name_A', 'last_name_A').values('first_name_A', 'last_name_A').distinct()
queryB = tableB.objects.order_by('first_name_B', 'last_name_B').values('first_name_B', 'last_name_B').distinct()

query = sorted(chain(queryA, queryB))

return render_to_response('results.html', {'queries': query})

html

<div>
{% for query in queries %}
    <tr>
        <td>{{ query.first_name_A }} {{ query.last_name_A }}</td>
    </tr>
{% endfor %}
</div>

The above html will obviously only return the first and last names from tableA. These are sorted and distinct but no info from tableB is included. As the columns from each table have different headings and no properties alike other than the values (rather than the keys) how can I merge them, sort and select distinct combinations? Or should I just rename the columns to remove the table reference?

Answer 1

You can use the extra() modifier to assign alternative names.

queryA = tableA.extra(select={'alias' : 'first_name_A'}).order_by('first_name_A').values('alias').distinct()
queryB = tableB.extra(select={'alias' : 'first_name_B'}).order_by('first_name_B').values('alias').distinct()

The django documentation discourages the use of extra but there doesn't seem to be an easy way to create aliases in a queryset. This ticket suggest that this may change in the future. The last comment describes an alternative method with F expressions and annotate which should work with django v1.8

Answer 2

I've solved it but I'm not sure it's the most pythonic (nor correct) way. If anyone has any better suggestions then please let me know.

views.py

        queryA = tableA.objects.order_by('first_name_A','last_name_A').values('first_name_A', 'last_name_A').distinct()
        queryB = tableB.objects.order_by('first_name_B','last_name_B').values('first_name_B', 'last_name_B').distinct()


        chain_query = sorted(chain(queryA, queryB))


        valueList = []


        for q in chain_query:
            wholeName = ' '.join(q.values())
            valueList.append(wholeName)


        query = sorted(set(valueList))  

        return render_to_response('results.html', {'queries': query})

html

    {% for query in queries %}
    <tr>
        <td>{{ query }}</td>
    </tr>
    {% endfor %}