In it's simplest form, pagination in Grails is quite straight forward, but what happens when you deviate from the simple?  Dynamic finders to the rescue!

Problem:  Provide pagination for a subset of domain objects

Answer:  See the code below

Domain Object

class Cycle {
     String manufacturer
}

Controller

We need to be able to select cycles based upon their manufacturer (ie. Honda, Yamaha, Kawasaki, etc.).  The manufacturer we wish to query on will be included in the request as a parameter.

class CycleController {
    def index = {
        params.max = Math.min( params.max ? params.max.toInteger() : 16,  100)
        
            def cycleCount = Cycle.countByManufacturer(params.manufacturer)
            def cycles = Cycle.findAllByManufacturer(params.manufacturer, [max: params.max as Integer, offset: params.offset as Integer])

        [ cycleInstanceList: cycles, cycleInstanceTotal: cycleCount ]
    }
}

It's important to note the need to cast the values of parameters max and min to Integer.  If this is not done, max and min will not work for pagination.  I thought autoboxing would work in this case, but it doesn't.

The countBy* dynamic query is a great way to get counts without being resource intensive.  A countBy needs to be performed because calling size() on the cycles list will only give the size of the list (in this case 16) as limited by the max parameter.

index.gsp - Paginate Tag

The params tag is included so that when pagination buttons are clicked, the required manufactured parameter will be passed to the controller and used to limit the results to the desired manufacturer.

I hope this helps many of you with pagination.  Of course, things can always get more complicated, that's when critera queries are needed, but in most cases, dynamic finders will suffice.