My Sql Server database has some nullable nvarchar fields, and no nvarchar fields containing empty strings. I want to keep it this way, but the default MVC model binder seems to turn null strings into empty strings.
When a controller retrieves a null nvarchar database field, the null field turns into null string inside the controller, and from there the view renders them, say as blank text boxes. When the page is posted, the default model binder uses these blank text boxes to update the model, and the formerly null strings are changed to empty strings. When the data is updated back to the database, nulls are overwritten with empty strings.
What is the easiest way to get model binding to leave these nulls unchanged?
I know you are probably looking for something more sophisticated, but the default behavior of the ModelBinder is to convert empty form field values into the default value for the datatype of your model object property. String properties become empty, int properties become 0, etc.
You can obviously create a validation scheme that will check for string.empty and convert to null prior to updating the DB. For int form fields you will need to check for 0, and then convert to null.
Here's a hack I used a few months ago before I found the eden of stackoverflow. :) It's a pain, and doesn't scale well, but it works:
Basically, you override the binding inside of a partial linq object. If there's a value you know should always be null (but never legitimately empty) you can do the following. I used this for a string-based user id (SID).
partial void OnSubProcess_Owner_UserChanged()
{
if (string.IsNullOrEmpty(this.SubProcess_Owner_User))
this._SubProcess_Owner_User = null;
}
James
The right answer might be to override the default Model binder to add this functionality yourself.
Maybe you could have a NullValueAttribute that you could apply to string properties to identify the null value. Then make empty string a null value.
I am experiencing the same problem at the moment and will probably resort to this
I'm posting this answer to follow through on this question. After working with it for a while I came to see this problem as part of the general concern of model integrity. For a while I had implemented a solution inside my update stored procedures to catch empty strings and turn them to nulls, along the lines of mikerennick's answer above. Later I wanted also to make sure fields were trimmed and I happened to move the application to NHibernate (and most of the stored procedures went away). In the end I embedded some POCO logic to trim and check for empty strings (from whatever source) in the setters as so:
public MyClass {
private string _name;
public string Name {
get { return _name; }
set { _name = value.TrimToNullIfEmpty(); }
}
}
public static class StringExtensions {
public static string TrimToNullIfEmpty(this string s) {
string temp = (s ?? "").Trim();
return temp.Length == 0 ? null : temp;
}
}
Related
So I have a class with a property like this:
public class Foo
{
[Column("GBBRSH")
public static string Gibberish { get; set;}
....
}
For saving data, I have it configured so that the update/insert statements use a custom function:
public static string GetTableColumnName(PropertyInfo property)
{
var type = typeof(ColumnAttribute);
var prop = property.GetCustomAttributes(type, false);
if (propr.Count() > 0)
return ((ColumnAttribute)prop.First()).Name;
return property.Name;
}
This handles fine, but I noticed that when I go to retrieve the data, it isn't actually pulling data back via the function for this particular column. I noticed that the other data present was pulled, but the column in question was the only field with data that didn't retrieve.
1) Is there a way to perhaps use the GetTableColumnName function for the retrieval part of Dapper?
2) Is there a way to force Dapper.NET to throw an exception if a scenario like this happens? I really don't want to have a false sense of security that everything is working as expected when it actually isn't (I get that I'm using mapping that Dapper.NET doesn't use by default, but I do want to set it up in that manner).
edit:
I'm looking in the SqlMapper source of Dapper and found:
private static IEnumerable<T> QueryInternal<T>(params) // my knowledge of generics is limited, but how does this work without a where T : object?
{
...
while (reader.Read())
{
yield return (T)func(reader);
}
...
}
so I learned about two things after finding this. Read up on Func and read up on yield (never used either before). My guess is that I need to pass reader.Read() to another function (that checks against column headers and inserts into objects appropriately) and yield return that?
You could change your select statement to work with aliases like "SELECT [Column("GBBRSH")] AS Gibberish" and provide a mapping between the attribute name and the poco property name.
That way, Dapper would fill the matching properties, since it only requires your POCO's to match the exact name of the column.
I needed a domain class that held a list of Strings. It seems fairly well-known that GORM can't handle this, so I've worked around it. At first I tried using getters and setters in the domain class, but that caused problems. Then I found on Stack Overflow a way to use afterLoad() and beforeValidate() to rewrite properties as shown below. This has worked well to allow me to turn the List into a String for persistence and back to a List for use in the app.
class Entries {
// persisted to database
String _entry
// exposed to app
List entry
static transients = ['entry'] //don't try to persist the List
def afterLoad() {
// split the String from the database into a List
entry = _entry?.split('\\|')
}
def beforeValidate() {
// join the List into a String for persisting
_entry = entry.join('|')
}
static constraints = {
_entry maxSize:4000
}
}
This works fine programmatically. The only problem is that the Grails scaffolding can't deal with this, even if I try to enter a pipe-delimited string. I understand the reason why is that the scaffolding creates a form field for _entry, so entry is null when it tries to save the object. And beforeValidate() relies on a List of Strings to work.
I tried to get around this in the controller, by setting params.entry = params._entry, prior to the call to new Entries(params). [I recognize that this is not a perfect solution, but this was my first pass at getting the form working.] And then I added a test in beforeValidate() to set entry = _entry if entry was null. Basically:
EntriesController.groovy:
params.entry = params._entry // I added this line
def entriesInstance = new Entries(params)
Entries.groovy:
def beforeValidate() {
if( entry == null ) entry = _entry // I added this line
_entry = entry.join('|')
}
I thought that would allow me to enter pipe-delimited strings into the scaffolded Create Entries form and get something into the database.
To my surprise, though, I found that both entry and _entry were null in beforeValidate(), even though I printed and verified that params contained both keys in the controller. I don't understand why this happens. How did my adding a new key to params result in nulls arriving in the domain class?
The follow-up question is, of course, what's the right way to make the scaffolded Create Entries form accept a pipe-delimited String that makes it into the database?
I needed a domain class that held a list of Strings. It seems fairly well-known that GORM can't handle this, so I've worked around it.
I don't agree with you here
class Xyz {
static hasMany = [entries: String]
}
Should create a seperate table to hold your list of strings (It will actually be a Set). Here are the docs
I am trying to realize valition on data type. I have used DataAnnotations, but for data type it's not showing customized message
for example when I' am trying enter string data into int typed field. How I can customize messages in this case?
If I had to guess, you sound like you want a custom message to display when validating one or more fields in your model. You can subclass the DataAnnotations.ValidationAttribute class and override the IsValid(object) method and finally setting a custom ErrorMessage value (where ErrorMessage already belongs to the ValidationAttribute class)
public class SuperDuperValidator : ValidationAttribute
{
public override bool IsValid(object value)
{
bool valid = false;
// do your validation logic here
return valid;
}
}
Finally, decorate your model property with the attribute
public class MyClass
{
[SuperDuperValidator(ErrorMessage="Something is wrong with MyInt")]
public int MyInt { get; set; }
}
If you're using out-of-the-box MVC3, this should be all you need to propertly validate a model (though your model will probably differ/have more properties, etc) So, in your [HttpPost] controller action, MVC will automagically bind MyClass and you will be able to use ModelState.IsValid to determine whether or not the posted data is, in fact, valid.
Pavel,
The DataAnnotations DataType attribute does not affect validation. It's used to decide how your input is rendered. In such a case, David's solution above works.
However, if you want to use only the built-in validation attributes, you probably need to use the Range attribute like this:
[Range(0, 10, ErrorMessage="Please enter a number between 0 and 10")]
public int MyInt { get ; set ;}
(Of course, you should really be using the ErrorMessageResourceName/Type parameters and extract out hard-coded error message strings into resx files.)
Make sure to let MVC know where to render your error message:
<%= Html.ValidationMessageFor(m => m.MyInt) %>
Or you can just use EditorForModel and it will set it up correctly.
I don't think this has been answered because I have the same issue.
If you have a Model with a property of type int and the user types in a string of "asd" then the MVC3 framework binding/validation steps in and results in your view displaying "The value 'asd' is not valid for <model property name or DisplayName here>".
To me the poster is asking can this message that the MVC3 framework is outputting be customized?
I'd like to know too. Whilst the message is not too bad if you label your field something that easily indicates an number is expected you might still want to include additional reasons so it says something like:
"The value 'asd' is not valid for <fieldname>; must be a positive whole number."
So that the user is not entering value after value and getting different error messages each time.
I may be out of date, but one principle I adhere to is avoid nulls as much as possible.
However what I have found is that for a strongly typed view in which the user inputs the properties of an object I want to save, if some fields are not entered they are assigned as null.
Then when you try to save the changes, the validation fails.
So rather than set each property to an empty string, how can I automatically set each TextBox on a form to default to an empty string rather than a null?
You could put the following attribute on your string-properties in your model:
[DisplayFormat(ConvertEmptyStringToNull=false)]
So whenever someone posts a form with empty text-fields, these will be an empty string instead of null...
To be honest, I'd say your coding methodology is out of date and flawed. You should handle all possibilities, it's not hard. That's exactly what string.IsNullOrEmpty(value); is for.
I'm guessing your validation logic is something like:
if (value == string.Empty) { isValid = false; }
So it doesn't handle the null values. You should replace that check so it also checks for nulls.
string value1 = null;
string value2 = string.Empty;
string.IsNullOrEmpty(value1); // true
string.IsNullOrEmpty(value2); // true
An alternative solution to using attributes on each model property, as described in the accepted answer, is using a custom model binder, see string.empty converted to null when passing JSON object to MVC Controller
I ran across this problem when dealing with an old service that requires empty strings. I created an extension method:
public static string GetValueOrDefault(this string str)
{
return str ?? String.Empty;
}
So you can use this when you want to make sure any strings that are null become empty
yourString1.GetValueOrDefault();
yourString2.GetValueOrDefault();
To put it in basic form, my database table doesn't allow nulls for varchars, it must have blanks. My model doesn't allow nulls so it won't insert a record if I leave form fields empty. If an empty form field appears I want a default value of blank to be used instead. I've tried, for example, the following without any luck:
[Column]
[DisplayName("WMD Company")]
[DefaultValue(" ")]
public string WMDCompany { get; set; }
So instead, in my controller action I have to do a check like the following:
if(myModel.WMDCompany == null) myModel.WMDCompany = " ";
Which is plain nasty to me. Is there any way of getting [DefaultValue(" ")] to work?
Cheers
What about something like this:
private string wmdCompany;
public string WMDCompany
{
get
{
if (this.wmdCompany == null)
{
return string.Empty;
}
return this.wmdCompany;
}
set
{
this.wmdCompany = value;
}
}
The DefaultValue attribute is not used. LINQ to SQL has not support for DB defaults unfortunately. That property is intended for use in API extension if I remember, but I don't know of any that use it.
Two approaches to get around this you could use.
First update your data layer, by appropriately controlling the property, and setting it to null. Use a partial class to extend your data class, and implement the OnCreated() partial method, and in this set the value to String.Empty.
partial void OnCreated()
{
MyProp = String.Empty;
}
Secondly, you could change your DBML representation to allow nulls, but in your database, use a trigger to convert NULLs to empty strings.
I'd go with the first approach myself - assuming you can't just use NULLs as suggested by Adrian
Inserting spaces as a placeholder for NULL seems like a very obscure method to me. Why don't you just change your table design to allow NULL values?