Defining and Using Literal Properties

Defining Literal Properties

To add a literal property to a class definition, add an element like one of the following to the class:

Property PropName as Classname;

Property PropName as Classname [ Keywords ] ;

Property PropName as Classname(PARAM1=value,PARAM2=value) [ Keywords ] ;

Property PropName as Classname(PARAM1=value,PARAM2=value) ;

Where:

• PropName is the name of the property.

• Classname is the class on which this property is based. If you omit this, Classname is assumed to be %StringOpens in a new tab. To define this property as a literal property, either omit Classname or specify Classname as the name of a data type class; see “Common Data Type Classes” later in this chapter. You could also use a custom data type class.

• Keywords represents any property keywords. See “Compiler Keywords,” earlier in this book. Later sections of this chapter discuss additional keywords.

• PARAM1, PARAM2, and so on are property parameters. The available parameter depend on the class used by the property. Later sections of this chapter lists the most common property parameters.

Notice that the property parameters, if included, are enclosed in parentheses and precede any property keywords. The property keywords, if included, are enclosed in square brackets.

Examples

For example, a class can define a Count property using the %IntegerOpens in a new tab data type class:

Property Count As %Integer;

Because %IntegerOpens in a new tab is a data type class, Count is a data type property.

You can use data type parameters to place constraints on the allowed values of data type properties. For example, for a property of type %IntegerOpens in a new tab, you can specify the MAXVAL parameter:

Property Count As %Integer(MAXVAL = 100);

To set a data type property value equal to the empty string, use code of the form:

 Set object.Property = $C(0)

Every property has a collation type, which determines how values are ordered (such as whether capitalization has effects or not). The default collation type for strings is SQLUPPER. For more details on collations, see “Data Collation” in the chapter “Caché SQL Basics” in Using Caché SQL.

Defining an Initial Expression for a Property

By default, when a new object is created, each property equals null. You can specify an ObjectScript expression to use as the initial value for a given property instead. The expression is evaluated when the object is created.

To specify an initial expression for a property, include the InitialExpression keyword in the property definition:

Property PropName as Classname [ InitialExpression=expression ] ;

Where expression is an ObjectScript expression (note that you cannot use any other language for this expression). For details, see InitialExpression in the Caché Class Definition Reference.

Defining a Property As Required

This option applies only to properties in persistent classes. (Note that this option applies to any kind of property, not just literal ones.)

By default, when a new object is saved, Caché does not require it to have non-null values for its properties. You can define a property so that a non-null value is required. To do so, include the Required keyword in the property definition:

Property PropName as Classname [ Required ] ;

Then, if you attempt to save an object that has a null value for the property, Caché issues the following error:

ERROR #5659: Property required

The Required keyword also affects how you can insert or modify data for this class via SQL access. Specifically, an error occurs if the value is missing when you attempt to insert or update a record.

Defining a Computed Property

In Caché, you can define computed properties, whose values are computed via ObjectScript, possibly based on other properties. The generic phrase computed properties (or computed fields) includes both of the following variations:

• Always computed — The value of this property is calculated when it is accessed. It is never stored in the database.

• Triggered computed — The value of this property is recalculated when triggered (details given below).

  If the property is defined in a persistent class, its value is stored in the database.

In both cases, the recalculation is performed whether you use object access or an SQL query.

There are five property keywords (SqlComputed, SqlComputeCode, SqlComputeOnChange, Transient, Calculated) that control if and how a property is computed. The following table summarizes the possibilities:

To define a computed property, do the following:

• Include the SqlComputed keyword in the property definition. (That is, specify the SqlComputed keyword as true.)

• Include the SqlComputeCode keyword in the property definition. For the value of this keyword, specify (in curly braces) a line of ObjectScript code that sets the value of the property, according to rules given in “SqlComputeCode” in the reference “Property Keywords” in Caché Class Definition Reference. For example:

  Property FullName As %String [ SqlComputeCode = {set {*}={FirstName}_" "_{LastName}}, SqlComputed ];
  

  

• If you want to make the property always computed, specify the Calculated keyword as true in the property definition.

  Or, if you want to make the property triggered computed, do not include the Calculated and Transient keywords in the property definition. (That is, make sure both keywords are false.)

• If the property is triggered computed, optionally specify SqlComputeOnChange.

  This keyword can specify one or more properties. When any of these properties change in value, the triggered property is recomputed. Note that you must use the property names rather than the names given by SqlFieldName, which is discussed later in this chapter). For example (with artificial line breaks):

  Property messageId As %Integer [ 
  SqlComputeCode = { set {*}=$Select({Status}="":0,1:$List($List($Extract({Status},3,$Length({Status}))))) }, 
  SqlComputed, SqlComputeOnChange = Status ];
  

  

  For another example (with artificial line breaks):

  Property Test2 As %String [ SqlComputeCode = { set {*}={Refprop1}_{Refprop2}}, SqlComputed, 
  SqlComputeOnChange = (Refprop1, Refprop2) ];
  

  

  The value of SqlComputeOnChange can also include the values %%INSERT or %%UPDATE; for details, see SqlComputeOnChange.

If you intend to index this field, use deterministic codeOpens in a new tab, rather than nondeterministic code. Caché cannot maintain an index on the results of nondeterministic code because it is not possible to reliably remove stale index key values. (Deterministic code returns the same value every time when passed the same arguments. So for example, code that returns $h is nondeterministic, because $h is modified outside of the control of the function.)

Also see the Calculated keyword in the Caché Class Definition Reference. And see “Controlling the SQL Projection of Computed Properties,” later in this chapter.

Defining a Multidimensional Property

You can define a property to be multidimensional, which means that you intend the property to act as a multidimensional array. To do so, include the MultiDimensional keyword in the property definition:

Property PropName as Classname [ MultiDimensional ] ;

This property is different from other properties as follows:

• Caché does not provide property methods for it (see “Using and Overriding Property Methods,” later in this book).

• It is ignored when the object is validated or saved.

• It is not saved to disk, unless your application includes code to save it specifically.

• It cannot be exposed through ActiveX or Java.

• It cannot be stored in or exposed through SQL tables.

Multidimensional properties are rare but provide a useful way to temporarily contain information about the state of an object.

Also see “Specifying Values for a Multidimensional Property,” later in this chapter.

Common Data Type Classes

Caché provides a large number of data type classes. Of these, the classes in most common use are as follows:

There are many additional data type classes, intended for specialized uses; most of these types are subclasses of the classes listed here. See the InterSystems Class Reference for details.

Each data type class specifies several keywords that control how that data type is used in Caché SQL and how the data type is projected to client systems:

• SqlCategory — Specifies the SQL category to use for the data type when the Caché SQL engine performs operations upon it.

• OdbcType — Specifies the ODBC type used when the data type is accessed via ODBC.

• ClientDataType — Specifies the Java or ActiveX type used when the data type is accessed via client applications.

Therefore, when you choose a data type class, you should choose a class that has the appropriate client projection, if applicable, for your needs. Use the following subsections to help choose a suitable data type class. If there is no suitable class, you can create your own data type class, as described in the chapter “Defining Data Type Classes.”

Core Property Parameters

Depending on the property, you can specify parameters of that property, to affect its behavior. For example, parameters can specify minimum and maximum values, formatting for use in display, collation, delimiters for use in specific scenarios, and so on. You can specify parameters either in the Inspector or by directly typing into the class definition. The following shows an example:

Property MyProperty as MyType (MYPARAMETER="some value");

Some parameters are available for all properties, of any type. These parameters are as follows:

• CALCSELECTIVITY — Controls whether the Tune Table facility calculates the selectivity for a property. Usually it is best to leave this parameter as the default (1). For details, see “Tune Table” in the Caché SQL Optimization Guide.

• CAPTION — Caption to use for this property in client applications.

• EXTERNALSQLNAME — Used in linked tables, this parameter specifies the name of the field in the external table to which this property is linked. The Link Table wizard specifies this parameter for each property when it generates a class. The name of the SQL field on the remote database may need to differ from property name on the Caché server for various reason, such as because the remote database field name is a reserved word in Caché. For information on linked tables, see “The Link Table Wizard” in Using Caché SQL.

  Note that the property parameter EXTERNALSQLNAME has a different purpose than the SQLFieldName compiler keyword, and these items can have different values. SQLFieldName specifies the projected SQL field name in the Caché database, and EXTERNALSQLNAME is the field name in the remote database.

• EXTERNALSQLTYPE — Used in linked tables, this parameter specifies the SQL type of the field in the external table to which this property is linked. The Link Table wizard specifies this parameter for each property when it generates a class. See EXTERNALSQLNAME.

• JAVATYPE — The Java data type to which this property is projected.

Most property parameters are defined in data type classes and thus are class-specific; see the next section.

Class-Specific Property Parameters

The previous section lists the parameters that are available for all properties. The other available parameters depend on the class used by the property. The following table lists the parameters supported by the data type classes listed in this chapter. The parameters are grouped into three columns: 1) parameters that are found in many data type classes or that are otherwise commonly encountered, 2) parameters that have meaning only in XML and SOAP contexts, and 3) parameters that occur in only a few data type classes and that are rarely encountered.

*This special-purpose class is only for use in migrations to Caché.

Defining Enumerated Properties

Many properties support the parameters VALUELIST and DISPLAYLIST. You use these to define enumerated properties.

To specify a list of valid values for a property, use its VALUELIST parameter. The form of VALUELIST is a delimiter-separated list of logical values, where the delimiter is the first character. For instance:

Property Color As %String(VALUELIST = ",red,green,blue");

In this example, VALUELIST specifies that valid possible values are “red”, “green”, and “blue”, with a comma as its delimiter. Similarly,

Property Color As %String(VALUELIST = " red green blue");

specifies the same list, but with a space as its delimiter.

The property is restricted to values in the list, and the data type validation code simply checks to see if the value is in the list. If no list is present, there are no special restrictions on values.

DISPLAYLIST is an additional list that, if present, represents the corresponding display values to be returned by the LogicalToDisplay() method of the property.

For an example that shows how to obtain the display values, see the section “Using Property Methods,” later in this chapter.

Specifying Values for Literal Properties

To specify a value for a literal property, use the SET command, an OREF, and dot syntax as follows:

 SET oref.MyProp=value

Where oref is an OREF, MyProp is a property of the corresponding object, and value is an ObjectScript expression that evaluates to a literal value. For example:

 SET patient.LastName="Muggles"
 SET patient.HomeAddress.City="Carver"
 SET mrn=##class(MyApp.MyClass).GetNewMRN()
 set patient.MRN=mrn

The literal value must be a valid logical value for the property type. For example, use 1 or 0 for a property based on %BooleanOpens in a new tab. For another example, for an enumerated property, the value must be one of the items specified by the VALUELIST parameter.

Specifying Values for a Multidimensional Property

For a multidimensional property, you can specify values for any subscripts of the property. For example:

 set oref.MyStateProp("temp1")=value1
 set oref.MyStateProp("temp2")=value2
 set oref.MyStateProp("temp3")=value3

Multidimensional properties are useful for holding temporary information for use by the object. These properties are not saved to disk.

Using Property Methods

Each property adds a set of generated class methods to the class. These methods include propnameIsValid(), propnameLogicalToDisplay(), propnameDisplayToLogical(), propnameLogicalToODBC(), propnameODBCToLogical(), and others, where propname is the property name. Some of these methods are inherited from the %Property class and others are inherited from the data type class on which the property is based. For details and a list of the methods, see the chapter “Defining Data Type Classes.”

Caché uses these methods internally, and you can call them directly as well. In each case, the argument is a property value. For example:

SAMPLES>set p=##class(DeepSee.Study.Patient).%OpenId(1)
 
SAMPLES>w p.Gender
M
SAMPLES>w ##class(DeepSee.Study.Patient).GenderLogicalToDisplay(p.Gender)
Male

Controlling the SQL Projection of Literal Properties

A persistent class is projected as an SQL table, as described earlier in this book. For that class, all properties are projected to SQL, aside from the following exceptions:

• Transient properties (but see the subsection “Controlling the SQL Projection of Computed Properties”)

• Calculated properties (but see the subsection “Controlling the SQL Projection of Computed Properties”)

• Private properties

• Multidimensional properties

This section discusses the details for literal properties.

Specifying the Field Names

By default, a property (if projected to SQL) is projected as an SQL field with the same name as the property. To specify a different field name, use the property keyword SqlFieldName. (Note that it is necessary to use this keyword if the property name is an SQL reserved word). For instance, if there is a %StringOpens in a new tab property called “select,” you would define its projected name with the following syntax:

Property select As %String [ SqlFieldName = selectfield ];

If the name of a property is an SQL reserved word, you need to specify a different name for its projection.

Specifying the Column Numbers

The system automatically assigns a unique column number for each field. To control column number assignments, specify the property keyword SqlColumnNumber, as in the following example:

Property RGBValue As %String [ SqlColumnNumber = 3 ];

The value you specify for SqlColumnNumber must be an integer greater than 1. If you use the SqlColumnNumber keyword without an argument, Caché assigns a column number that is not preserved and that has no permanent position in the table.

If any property has an SQL column number specified, then Caché assigns column numbers for the other properties. The starting value for the assigned column numbers is the number following the highest SQL column number specified.

The value of the SqlColumnNumber keyword is inherited.

Effect of the Data Type Class and Property Parameters

The data type class used by a given property has an effect on the SQL projection. Specifically, the SQL category of the data type (defined with the SqlCategory keyword) control how the property is projected. Where applicable, the property parameters also have an effect:

For example, consider the following property definition:

Property Name As %String(MAXLEN = 30);

This property is projected as a string field with a maximum length of 30 characters.