问题
I'm trying to create derived properties based on contained objects.
Example below:
class Generation {
String name
DateTime productionStart
DateTime productionEnd
static belongsTo = [line: Line]
static hasMany = [bodyStyles: BodyStyle, engines: Engine, models: Model]
static constraints = {
line nullable: false
name nullable: false, unique: ['line'], maxSize: 255, blank: false
}
static mapping = {
// I've tried but this solution causes errors
productionStart formula: 'MIN(engines.productionStart)'
// I've tried but this solution causes errors
productionEnd formula: 'MAX(engines.productionEnd)'
}
}
class Engine {
String name
Integer horsePower
DateTime productionStart
DateTime productionEnd
static belongsTo = [generation: Generation]
static hasMany = [models: Model]
static constraints = {
generation nullable: false
name nullable: false, unique: ['generation', 'horsePower'], maxSize: 255, blank: false
horsePower nullable: false
productionStart nullable: false
productionEnd nullable: true
}
static mapping = {
productionStart type: PersistentDateTime
productionEnd type: PersistentDateTime
}
}
I've readed Derived Properties Documentation but my case is a little bit more complicated than formulas not associated with complex objects.
The solution that you can find in the code above results in an error::
Caused by GrailsTagException: Error executing tag : Error evaluating expression [Generation.findAll()] on line [23]: could not execute query; SQL [select this_.id as id22_0_, this_.version as version22_0_, this_.line_id as line3_22_0_, this_.name as name22_0_, MAX(engines.productionEnd) as formula0_0_, MIN(engines.productionStart) as formula1_0_ from generation this_]; nested exception is org.hibernate.exception.SQLGrammarException: could not execute query
回答1:
Another way to try it is to create a getter instead of derived properties:
class Generation {
String name
DateTime productionStart
DateTime productionEnd
static transients = ['productionStart','productionEnd']
static belongsTo = [line: Line]
static hasMany = [bodyStyles: BodyStyle, engines: Engine, models: Model]
static constraints = {
line nullable: false
name nullable: false, unique: ['line'], maxSize: 255, blank: false
}
DateTime getProductionStart() {
def datetime = Engine.createCriteria().get {
eq('generation',this)
projections {
min('productionStart')
}
}
return datetime
}
DateTime getProductionEnd() {
def datetime = Engine.createCriteria().get {
eq('generation',this)
projections {
max('productionEnd')
}
}
return datetime
}
}
来源：https://stackoverflow.com/questions/19164445/derived-properties-using-aggregate-functions-in-grails