public protocol Model {
}

open class FindRequest<T: RecordTemplate, MD: RecordTemplate>: Request {
}

open class Request {
  open func cacheKey() -> String {
    return ""
  }
}

open class CollectionResponse<V: RecordTemplate, MD: RecordTemplate> {
}

open class RecordTemplate {
}

// Another file:

func extractCollectionModel<ModelType : Model , MetadataType: Model>(_ response: CollectionResponse<ModelType, MetadataType>?) {
    
}

extension RecordTemplate : Model {
}


public protocol CachableRequest {
    /**
     Customized cacheKey to return alternative cache key if specified
     */
    func ndlCacheKey() -> String

    /**
     This is implmented by the Request class.
     */
    func cacheKey() -> String
}

extension FindRequest: CachableRequest {
}

// MARK: Implementation

extension CachableRequest {
    /**
     customized cacheKey to return alternative cache key if specified
     */
    public func ndlCacheKey() -> String {
        return cacheKey()
    }
}

func extractCollectionModel<ModelType : Model , MetadataType: Model>(_ response: CollectionResponse<ModelType, MetadataType>?) {
    
}

extension RecordTemplate : Model {
}

// ModelType and MetadataType are already constraint to be of protocol type Model because CollectionResponse's generic parameters are constraint to be RecordTemplate.
func extractCollectionModel<ModelType , MetadataTypel>(_ response: CollectionResponse<ModelType, MetadataType>?) {
    
}

extension RecordTemplate : Model {
}