How to filter records in a form by code

The standard filter functionality in Ax forms is a neat and powerful feature.
Using this filter functionality in your code is something you'll definitely use at some point in time as a programmer.

Although it's possible to do it in a single line of code, I prefer a 3 step solution. That way it's more flexible.
Let me show you by example. We'll filter the customers records in form CustTable, only showing customers with currency USD.

Step 1: Declare a class variable
In the ClassDeclaration method of the form, define a range.

QueryBuildRange CurrencyQBR;

Step 2: Instantiate the new range.
In the init method on the datasource of the form, you assign the range to a specific field (after the super call).

public void init()
{
super();

CurrencyQBR = this.query().dataSourceName('CustTable').addRange(fieldnum(CustTable,Currency));
}
Step 3: In the last step, you assign a value to the range.
This is done in the executeQuery method on the same datasource of the form. Before the super call. Like this:

public void executeQuery()
{ ;

CurrencyQBR.value(queryvalue('USD'));

super();
}
You're done! When you open the form, your customer records are filtered, you only get the customers with currencycode USD set up.

Like I said in the intro of this post, this can be done in one line of code as well.
In the init method of the form datasource, after the super call, place this code:

this.query().dataSourceName('CustTable').addRange(fieldnum(CustTable,Currency)).value(queryvalue('USD'));
But this way, it's fixed. If you choose the 3 step method, you could for example use a variable in the range value. The way to go would be to place an input field on your form, get the value from it and supply it in the executeQuery method.

For example like this:

public void executeQuery()
{ ;

CurrencyQBR.value(queryvalue(MyInputField.text()));

super();
}
Just make sure the executeQuery method is executed, thus applying the desired filter (maybe be using a button on your form to activate it).
Of course it's possible to combine multiple querybuildranges.