Validators between classical and professional usage

April 29, 2010

Web2py validators work at the form level, not on the database level
It's recommended though to make both validation at the form level and database level.

  • You can add validators whereever you want but you’ve to pay attention for the logic.
    you can make:

    db.define_table('my_table', Field('my_f', requires=[..]))
    # or  inside the same model after the definition of the table :
    db.define_table('my_table', Field('my_f'))
    db.my_table.my_f.requires = [....]
     # you may even do this in your controller
    db.my_table.my_f.requires = [....]  
    form = SQLFORM(db.my_table)

    Things to note though that making adding validators after defining the table or before defining the form in your controller is that in those 2 particular situations, you’re really overriding any other validators on this table field defined in the model, either within the table definition or after it .

    db.define_table('my_table', Field('my_f', requires=[set_no_1]))
    db.my_table.my_f.requires = [set_no_2]  

    In the example above, set_no_2 of validators is the one that will actually work.

    To overcome this situation, you may make something like:

    db.define_table('soso', Field('lolo'), Field('moon', requires=[IS_NOT_EMPTY()]))
    db.soso.moon.requires.append(IS_NOT_IN_DB(db, db.soso.moon))

    Wow !!! then you can append to the list from anywhere and it will work.

  • Now one can wonders , so what’s the catch ?

  • The catch is that making something like :
    db.my_table.my_f.requires = [IS_IN_DB(...)]

    will prevent this field from being represented as a combo box, for this to be achieved you’ve not to make a list

    db.my_table.my_f.requires = IS_IN_DB(...)

    in this case you can’t add other validators and any trial to override the validators will ruin the combo box widget.

  • The good news is that there’s a solution 😀 which is ‘using widgets’, look ath this piece of code:
    b.define_table('soso', Field('lolo'), Field('moon', widget=SQLFORM.widgets.options.widget, requires=[IS_IN_DB(db,, IS_UPPER()]))

    What I did was to force the usage of the options widget, and in this case it will work fine and the other validator will also work, and the value to be entered in database is the the uppercase value of the one you’ve chosen.
    WARNING: IS_IN_DB() MUST be the first in the list, for this to work.

  • IS_IN_DB() and IS_NOT_IN_DB() logical issue

    when using either of the 2 validators, you have to take care of something , which is that you can’t do something like:

    db.define_table('country', Field('name', requires=IS_NOT_IN_DB(db,

    what you want to do is making the country name unique, and not to allow duplication.
    Let’s look more precisely about what you’ve done above.
    can you really use before the country table is really defined ? You can’t and thus in this case you’ve to move your validation to be after the table definition , something like:

    db.define_table('country', Field('name')) IS_NOT_IN_DB(db,

    IS_IN_DB() or IS_IN_DB() classical usage
    The same logic goes for both, so I’ll concentrate on one the other will be almost the same, except for the functionality ofcourse.

    db.define_table('category', Field('name'))
    db.define_table('recipe', Field('category', 'reference category'))
       # or

    The parameters are:

    • a set of records
    • ‘’ (string) or (not a string) describes how the field will be represented in the
      database [this is the value that will be actually sent when accepting a SQLFORM that is using this table]
    • ‘’ describes how the field will be represented to the
      user [Just a representation] and you’ll end up with a combo box displaying the name while accepting the form will send the id to database.
      name is a valid field name in the same table.
    # model
    db.define_table('table_one', Field('name'))
    db.define_table('table_two',Field('a', requires=IS_IN_DB(db, '', '%(name)s')))
    # controller
    def index():
        form = SQLFORM(db.table_two)
        if form.accepts(request.vars, session):       
            print form
        return dict(form=form)

    supposing table_one has only one record {id=1, name=’hamdy’}
    the form will display a drop down box with one item “hamdy”
    but upon accepting form the id (1) will actually be stored in db

    print form will give you something like:

    <form action="" enctype="multipart/form-data" method="post">
    <select class="string" id="table_two_a" name="a"><option value=""></option><option value="1">hamdy</option></select></form>

    In fact to be honest what really got to the database in table_two is just an id.
    To make things more professional you’ve to make this id refer to the actual record in tabl_one not just an id.
    To achieve this and test it , do something like:

    # add to your model :
    db.define_table('table_three',Field('a', 'reference table_one', requires=IS_IN_DB(db, '', '%(name)s')))
    # controller
    def index():
        form = SQLFORM(db.table_three)
        if form.accepts(request.vars, session):       
            print form
        return dict(form=form)

    Now when you go to the appadmin interface to check you database,
    you’ll find that the id in the record is a link to the actual record in the other table and obviously this’s what you wanted from the beginning a foreign key.
    In this case you can add to the field ‘a’ in table_one : ondelete=’CASCADE’ so that when the record in a deleted , all other records in other tables referencing it will be deleted automatically and this is a thing you WILL ALWAYS need, otherwise you’ll suffer a severe headache trying to trace unused records and delete them manually
    in separate database queries.

    IS_IN_DB() professional usage

    • making something like:
      db.define_table('table_two',Field('a', requires=IS_IN_DB(db('h%')), '')))

      you’ll get a combo box with names in your table that starts with ‘h’ .
      choosing one of them and submitting the form will submit its id
      you may even do something like:

      db.define_table('table_two',Field('a', requires=IS_IN_DB(db('h%')),                       '', 'Name: %(name)s, id: %(id)s')))

      Note that :

       'Name: %(name)s, id: %(id)s')

      is just a string that looks similar to ‘Name:….., id:….’ :

      Name:hamdy, id:1
      Name:aaaa, id:2

      and as I said before, submitting it will submit the id .

      The bottom line here :

      • You just can play with the presentation [how field looks like for user] while the data that will enter the database is different.
      • You can use a query inside IS_IN_DB(query, presentation) and the query is just logically representing a WHERE clause of database query

    Another Professional usage for IS_IN_DB()

  • How to use IS_IN_DB() to make records entered by a user are unique
    but in the same time those records can be entered again but by another user ?
    • Using the web2py example, we have 2 tables, “owners” and “dogs”. The “dogs” table has a many 2 one relation to owners.
      How can I create a constraint/ validator that allows the owner to give
      his dogs unique names? I cannot use IS_NOT_IN_DB because it is is
      database wide, I would like to have duplicate names in the table, but
      not allow duplicate names for a given user. Example, an owner called
      Voltron can have 2 dogs called “skipper” and “lassie” but not
      “skipper” and “skipper”, but another owner called ‘Optimus Prime’ could
      also have a dog called “skipper” or “lassie”
      Was that clear? Any ideas?


    What I wanted to show you in the previous section is that you can do things using many ways and to ensure you understand the :
    IS_IN_DB() and IS_NOT_IN_DB() and that they can be passed (db) as the first parameter or passed a set of records using db(my_query)
    then the 2nd argument is a database field that either written as :
    'my_table.my_field' as a string or db.my_table.my_field

    Yet another professional usage for IS_IN_DB()

    How to make a data set of some records in a table with appropriate permissions then requires user to choose among them ?

    # Model
    db.define_table('company', ......)
                    Field('company_id', 'reference company'),                 
                    ) = IS_IN_DB(db(auth.accessible_query(Permissions.OWNER,,, if auth.is_logged_in() else None
    # -> to display company name instead of company id

    If you’ve a user who is a company OWNER or several companies OWNER
    then when he ever wants to enter news for one or more companies that he owns, the form should allow him to choose among those companies and make him choose which one to enter news for.


    returns records from company db.table that the current logged in user [] has OWNER permission on them
    In fact permissions are set based on groups so what this code really does is to check whether the current user belongs to a group that has OWNER permissions on some records of the table and return those records

    the reason for requires=….. if auth.is_logged_in() else None is that when a user is not logged in the auth.user is not there so it’s None, and that causes the application to fail because is not there [None has no id attribute] so simply I made that simple check knowing that this table is used for forms that requires a user to login.

    Now time for another neat trick that web2py permits
    In fact, you can make validation on a field based on another’s field’s value that is submitted from a form !!! on the fly.
    Consider the following example:

    db.define_table('my_table', Field('category', requires=IS_IN_SET(['url_required', 'url_not_required'
                           Field('url', requires = IS_EMPTY_OR(IS_URL()) if request.vars.category == 'url_not_required' else IS_URL()

    Great .. .right ? 😀

  • In fact IS_IN_SET can be a list of say … records ids and in the same time having labels [aliases] to be human readable
    so making something like:

    db.define_table('my_table', Field('toto', requires = IS_IN_SET([1, 2, 3], labels=['one', 'two', 'three'])))

    will yield a dropdown menu with the values[ ‘one’, ‘two’, ‘three’] but in fact what is being sent to database is the integers (ids) [1, 2 or 3]

  • You can even use IS_IN_LIST() with dates, … say for example you’ve a field of type ‘datetime’ then you want users to enter specific dates in it, so you can do something like:
    db.define_table('my_table', Field('date', 'datetime'))[,10,12),,11,02),,11,27)])

    Now user will get a drop down menu of the specified dates .

  • Another neat usage for IS_EMPTY_OR() or IS_NULL_OR() [they’re the same] is when you have a table with a field that refers to another table.
    in this particular situation the referencing field must have a value that is equivalent to an existing record id on the referenced table.
    and because it’s difficult to remember the ids or even to know it, you should use IS_IN_DB() validator to have a drop down menu to choose from it . And as you all know, you can format the value in drop down menu to display names instead of ids and what is really going to be sent to server and database is the id.

    db.define_table('cat', Field('category'))
    db.define_table('my_table', Field('category', 'reference cat', requires=IS_IN_DB(db,
    # use 'reference cat' or without single quotes

    Now what if you want to have the db.my_table.category set to a category id or if there’s no category you just want Null in this database field or you want just a specific default id to be stored if user let this field empty.
    In this case you use : IS_EMPTY_OR(….., null=) so I can have something like :

    # controller
    def index():
       form = SQLFORM(db.my_table)
       if form.accepts(request.vars, session):
           response.vars = T('yes')
       return dict(form=form)
    db.define_table('cat', Field('category'))
    db.define_table('my_table', Field('category', 'reference cat', requires=IS_EMPTY_OR(IS_IN_DB(db,, null=None)))

    In this case you can just let this field empty and not to choose a category and in this case will be stored in this field.
    If you want something other than such as a default category id, you can use :

    requires=IS_EMPTY_OR(IS_IN_DB(db,, null=1))
    # 1 should be valid record in
    <strong>Warning:</strong> if the value assigned yo null= dosn't exist , you 'll get an error trying to access database table via appadmin interface, it should be a valid record id in the referenced table.

    oh, don’t forget to check my discussion about the IS_EMPTY_OR() validator in Web2py Zen

  • How could I get rid of some validators that I don’t like in some actions
    [controller functions] ?

    anywhere in your action before defining the SQLFORM, just do:


  • How could one validate against a set of translated values?

    you do it the normal way:

    db.define_table('table_two_two',Field('a', 'string', requires=IS_IN_SET([T('v'), T('d')])))
    #requires=[IS_IN_SET(..)] -&gt; No combobox
    #requires=[IS_IN_SET(..)] -&gt; combobox

    Date & Time Validators

    db.define_table('a', Field('date', 'datetime'))'%Y-%m-%d %H:%M:%S-%p', error_message=...)

    and the date field b will be processed in the desired format, specified by the custom format string that you can edit as you like :

    ('%Y-%m-%d %H:%M:%S-%p'

    For Numbers you use


  • For Telephones:
    you may use something like :

    # or for european something like
  • How do I validate a number that must always consist of 5 numerals including numbers starting with a 0? Here are examples are 12345, 01234, 89765. I used IS_INT_IN_RANGE but the numerals that start with 0 did not validate. ?

    You need to store as ‘string’ with length=5 and require


    Telephone numbers should be stored as strings otherwise numbers starting with 0 will be treated as octal numbers.

  • validation not working on FORM field why?

    Now when talking about FORM validation it’s the same as the SQLFORM validation , though you’ve to notice something :
    Take care and give form field a name in order for validation to work

    # validation here won't work, although the form submission seems to work just fine
    form = FORM(INPUT(_type='text', requires=IS_LENGTH(minsize=3, maxsize=8)),
        if form.accepts(request.vars, session):
            response.flash = T('hey')
        return dict(form=form)

    In order for the previous example to work, field needs to have a name
    so that requst.vars contains request.vars.field_name and the validation works.
    So you need change the form definition into something like:

    form = FORM(INPUT(_name='name', _type='text', requires=IS_LENGTH(minsize=3)),

  • You can use IS_LIST_OF() validator to validate a list of values

  • Q: I’ve my own function that does something complex and I want to use it for values submitted from a form to process that data and make some operation on it before actually being saved to database.
    What can I do?


    Suppose you’ve a function like:

    def my_owsome_func(x):
        return return x.replace('-', '_')

    Now I’m having in my model:

    db.define_table('my_table', Field('my_url', requires=IS_URL()))

    and in my controller:

     form = SQLFORM(db.my_table)
        if form.accepts(request.vars, session):
            response.flash = T('yes')
        return dict(form=form)

    Now I want when user inputs a URL, the my_owsome_func() processes the URL before actually saved into database then replace every occurrence of ‘-‘ with ‘_’.

    Think a little … what is the thing that processes data before actually being saved into database ?
    Yes … Validators
    Now if I were able to use my_owsome_func() in a validator, this would be awsome ..!!!!
    Yes you can :
    you can make your custom validator like:

    from gluon.validators import Validator
    class MY_CUSTOM_VALIDATOR(Validator):
        def __init__(self, f):
            self._f = f
        def __call__(self, value):
            return (self._f(value), None)

    as you can see a typical validator should (but not required to) extend the Validator class in gluon.validators .
    gluon.validators.Validator includes a formatter() function that you might need in formatting input.
    In fact few validators use formatter() any way, and you’re not going to use it at most conditions too.

    A typical validator includes __init__() and __call__() functions, where __init__() is called once validator class instantiated and is used to initialize the object .
    __call__() is called at the end of the execution and acts upon a value
    so validator needs to be called in this way :


    (value_to_check_against is saved as an attribute using the __init__()
    and value_to_check is called by __call__()

    A typical validator should return (value_to_check, None) if validation succeeded and (value_to_check, error_message) if not
    In our case we just made a validator returns (value_after_processing, None) … where None indicates validation succeeded
    We don’t need to have an error message because we use a validator to process data that should be correct .
    How can I guarantee that data is correct ?
    using another validator that is used before my custom one, so that we end up using one validator for data integrity and the other to process data before actually saving into database.

    In our example, we pass function to the validator and in the __call__() we use it to process data and return result.

  • CRYPT() validator

    Not used By default for password field in auth_user table [It should]
    to use it, You need to do somewhere in your model ,something like:


    auth.define_tables() is responsible for creating the default tables each application should have, one of them ofcourse is auth_user and thus :
    db.auth_user.password.requires=CRYPT() should come after it not before it.

    Doing that when you’re already having passwords saved in database means that they won’t work any more unless you made once and only once :

    for row in db().select(db.auth_user.ALL):