7.5.2.11 Derived Properties - Reference Documentation
Authors: Graeme Rocher, Peter Ledbrook, Marc Palmer, Jeff Brown, Luke Daley, Burt Beckwith, Lari Hotari
Version: 3.1.7
7.5.2.11 Derived Properties
A derived property is one that takes its value from a SQL expression, often but not necessarily based on the value of one or more other persistent properties. Consider a Product class like this:class Product { Float price Float taxRate Float tax }
tax
property is derived based on the value of price
and taxRate
properties then is probably no need to persist the tax
property. The SQL used to derive the value of a derived property may be expressed in the ORM DSL like this:class Product { Float price Float taxRate Float tax static mapping = { tax formula: 'PRICE * TAX_RATE' } }
PRICE
and TAX_RATE
instead of price
and taxRate
.With that in place, when a Product is retrieved with something like Product.get(42)
, the SQL that is generated to support that will look something like this:select product0_.id as id1_0_, product0_.version as version1_0_, product0_.price as price1_0_, product0_.tax_rate as tax4_1_0_, product0_.PRICE * product0_.TAX_RATE as formula1_0_ from product product0_ where product0_.id=?
tax
property is derived at runtime and not stored in the database it might seem that the same effect could be achieved by adding a method like getTax()
to the Product
class that simply returns the product of the taxRate
and price
properties. With an approach like that you would give up the ability query the database based on the value of the tax
property. Using a derived property allows exactly that. To retrieve all Product
objects that have a tax
value greater than 21.12 you could execute a query like this:Product.findAllByTaxGreaterThan(21.12)
Product.withCriteria { gt 'tax', 21.12f }
select this_.id as id1_0_, this_.version as version1_0_, this_.price as price1_0_, this_.tax_rate as tax4_1_0_, this_.PRICE * this_.TAX_RATE as formula1_0_ from product this_ where this_.PRICE * this_.TAX_RATE>?
Because the value of a derived property is generated in the database and depends on the execution of SQL code, derived properties may not have GORM constraints applied to them. If constraints are specified for a derived property, they will be ignored.