SqlComputeOnChange (Property Keyword)
Usage
To specify when a property is recomputed, use the following syntax:
Property name As classname [ SqlComputed, SqlComputeCode=sqlcomputecode, SqlComputeOnChange = propertynames ];
Where:
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sqlcomputecode specifies the code used the compute the property, as described in SqlComputeCode. If the compute code is defined in a PropertyComputation method instead, omit SqlComputeCode. For more details, see Computed Values.
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propertynames is either a single property name or a comma-separated list of property names. This value can also include the values %%INSERT or %%UPDATE. Note that you must use the actual property names, rather than the values given by SqlFieldname.
Details
This keyword applies only to triggered computed properties; it is ignored for other properties. (A triggered computed property is a property for which SqlComputed is true and SqlComputeCode is specified, but for which Calculated and Transient are both false. See “Defining a Computed Property” in Defining and Using Classes.)
This keyword controls the conditions under which this property is recomputed. Recomputation can result from:
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The modification of any specified properties.
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The occurrence of a triggering event.
If the keyword has a value of %%INSERT or %%UPDATE, then INSERT or UPDATE calls, respectively, specify event-triggered computation of the value of the field (property).
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With %%INSERT, InterSystems IRIS computes the field value when a row is inserted into the table. InterSystems IRIS invokes the code specified in the SQLComputeCode keyword to set the value. If SQLComputeCode uses the same field as an input value, then InterSystems IRIS uses the value explicitly provided for that field; if no value is given, InterSystems IRIS uses the InitialExpression (if this is specified) or null (if InitialExpression is not specified).
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With %%UPDATE, InterSystems IRIS computes the field value when a row is inserted into the table and recomputes it when a row is updated. In both cases, InterSystems IRIS invokes the code specified in the SQLComputeCode keyword to set the value. If SQLComputeCode uses the same field as an input value, then InterSystems IRIS uses the value explicitly provided for that field; if no value is given, InterSystems IRIS uses the previous field value.
Any event-triggered computation occurs immediately before validation and normalization (which themselves are followed by writing the value to the database).
In this example, property LastUpdate is updated with the current date and time when a row is inserted or updated:
Property LastUpdate As %TimeStamp [ SqlComputeCode = {set {*}=$ZDATETIME($NOW(),3)}, SqlComputed, SqlComputeOnChange = (%%INSERT, %%UPDATE) ];
Event-triggered computation of a field’s value may override any explicitly specified value for the property, depending on the code that computes the property’s value.
Default
The default value for the SqlComputeOnChange keyword is an empty string.
See Also
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“Property Definitions” in this book
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“Defining and Using Literal Properties” in Defining and Using Classes
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“Working with Collections” in Defining and Using Classes
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“Working with Streams” in Defining and Using Classes
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“Defining and Using Object-Valued Properties” in Defining and Using Classes
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“Defining and Using Relationships” in Defining and Using Classes
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“Introduction to Compiler Keywords” in Defining and Using Classes
