Storing the original filename and pre populating the form in web2py

autodelete

Upon deleting a record, the SQLFORM does not delete the physical uploaded file(s) that the record references. This is because web2py does not know if the same file is used/linked by other tables or used for a different purpose. You can do the following when the corresponding record is deleted and you are certain if it is safe to delete the actual file:

db.define_table('img_image',
    Field('n_name', requires=IS_NOT_EMPTY()),
    Field('source', 'upload', autodelete=True))

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By default, the autodelete attribute is set to False. When it is set to True, it ensures that the file is deleted if the record is deleted.

Links to referencing records

Consider the situation when two tables are linked by a reference field.

For example -

db.define_table('p_person',
    Field('n_name', requires=IS_NOT_EMPTY()))
db.define_table('dog',
    Field('owner', 'reference person'),
    Field('n_name', requires=IS_NOT_EMPTY()))
db.dog.owner.requires = IS_IN_DB(db, 'p_person.id', '%(n_name)s')

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Given information:

• A person has dogs
• Each dog belongs to an owner
• An owner is a person

The owner of dog is required to reference a valid db.p_person.id by '%(n_name)s'.
 

Let's add some people and their dogs using the appadmin interface for this application.

The appadmin UPDATE form displays a link to a page listing the dogs that belong to that person when editing an existing person. The SQLFORM's linkto argument can be used to simulate this behaviour. The URL of a new action that gets a query string from the SQLFORM and lists the related records must be pointed to by linkto.

For Example -

def display_form():
   record = db.p_person(request.args(0)) or redirect(URL('index'))
   link = URL('list_records', args='db')
   form = SQLFORM(db.p_person, record, deletable=True, linkto=link)
   if form.process().accepted:
       response.flash = 'form -> accepted'
   elif form.errors:
       response.flash = 'form has -> errors'
   return dict(form=form)

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Here is the page -

"dog.owner" is a link by using the labels argument of the SQLFORM, the name of this link can be changed.

For example -

labels = {'dog.owner':"This person's -> dogs"}

On clicking the link (dog.owner), you will be redirected to:

/test/default/list_records/db/dog?query=db.dog.owner%3D%3D3

"list_records" is the specified action with 

• request.args(0) set to the name of the referencing table
• request.vars.query set to the SQL query string

The URL's query string contains the appropriately URL-encoded (when the URL is parsed, web2py decodes this automatically ) value "dog.owner==3."

The "list records" action can be easily implemented as follows:

def list_records():
    import re
    REGEX = re.compile(r'^(\w+).(\w+).(\w+)==(\d+)$')
    m_match = REGEX.m_match(request.vars.query)
    if not m_match:
        redirect(URL('error'))
    table, field, id = m_match.group(2), m_match.group(3), m_match.group(4)
    records = db(db[table][field]==id).select()
    return dict(records=records)

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with the accompanying "default/list_records.html" view:

{{extend 'layout.html'}}
{{=records}}

Pre-populating the form

Pre-populating a form is always possible using the following syntax:

form.vars.name = 'field_value'

Regardless of whether the field (in this case, "name") is explicitly visualised in the form, statements like the one above must be added after the form declaration and before the form is accepted.

Frequently Asked Questions

What is web2py?

Web2py is a web application framework which is free and open-source, written in the Python programming language.

Why web2py?

Users can learn easily server-side web development, and it is lightweight and speedy.

What is Database Abstraction Layer?

A database abstraction layer is an API (Application Programming Interface) which unifies the communication between a computer application and databases such as Oracle, SQL Server, MySQL, IBM Db2 and PostgreSQL.

What is API?

API (Application Programming Interface) is a means of communication between two or more computer programs. It is a kind of software interface that provides a service to other software programs.

What is meant by Pre populating?

Pre populating means entering/filling in data in advance or, technically, populating form fields or databases in advance.

Conclusion

In this article, we discussed storing the original filename, autodelete attribute, links to referencing records and pre populating the form in web2py. We learnt where web2py keeps the original filename when downloaded, what happens upon deleting a record, what syntax to use to ensure that the file is deleted if the record is deleted, how to deal with situations where reference fields link tables and how to pre-populate a form.

To learn more about Web2Py, see Web2py, Web2Py init, Web2py Introduction, Web2Py Installation, Troubleshooting, and Application creation.

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