In this part, we would build the query module. This is in line with the CQRS principle: we separate the commands from the query.<\/p>\n
We would cover the following:<\/p>\n
<\/p>\n
Follow the steps below:<\/p>\n
Step 1<\/strong>: Add a kotlin file in the query package. Name it api. This file would hold the entity that would be stored in the database<\/p>\n Step 2<\/strong>: Annotate this class with the @Entity annotation<\/p>\n Step 3<\/strong>: Inside this file, create a data class with name CardSummary. The class will have the following parameters:<\/p>\n Step 4<\/strong>: Add the @id annotation to the id variable<\/p>\n The content of the api.kt file is as given in the listing below:<\/p>\n <\/p>\n <\/p>\n We would have to create a dataProvider in the GiftCardGUI class.This is required to create a projection that would be used in the grid.<\/p>\n Step 1<\/strong>: In the GiftCardGUI, create a final private variable of DataProvider (see code below)<\/p>\n Step 2<\/strong>: Do a constructor injection of a QueryGateway<\/p>\n At this point, the initial part of your GiftCardGUI would be the code listing below:<\/p>\n <\/p>\n <\/p>\n <\/p>\n Step 3:<\/strong> Place your cursor on the dataProvider(queryGateway) code which underlined with red squiggly lines.<\/p>\n Step 4:<\/strong> Choose ‘Create method dataProvider in GiftCardGUI’.<\/p>\n The method is create for you.<\/p>\n Step 5:<\/strong> Make this method return ‘new AbstractBackEndDataProvider. Make sure to use the intellisense to complete this statement.<\/p>\n You will see that two additional methods are added: fetchFromBackEnd() and sizeInBackEnd. We now need to implement these methods. Before write implement these method, we need to create a DataQuery and SizeQuery in the api.kt file<\/p>\n Step 6<\/strong>: Open the api.kt file (in the query module)<\/p>\n Step 7<\/strong>: Create a data class named DataQuery. This class should have two integer attributes offset and limit (see code below)<\/p>\n Step 8:<\/strong> Create another class (not data class) with name SizeQuery with no attributes.<\/p>\n The code for the two classes is shown below<\/p>\n <\/p>\n <\/p>\n Now back to the GiftCardGUI<\/p>\n Step 9<\/strong>: Now implement the two methods. The complete implementation is given below: <\/p>\n <\/p>\n We started by creating the entity class that would represent\u00a0 the data model (GiftCard)<\/p>\n Then we added a DataProvider variable to the GiftCardGUI<\/p>\n Then we did a construction injection of the DataProvider into the class<\/p>\n We then went to the api file and added DataQuery data class and a SizeQuery class<\/p>\n Finally we implemented the dataProvider method which returns a AbstractBackEndDataProvider object in which implemented the two required methods FetchFromBackEnd() and SizeInBackEnd().<\/p>\n <\/p>\n In the next part, we would then create the summaryProjection.<\/p>\n\n
@Entity\r\ndata class<\/span> CardSummary<\/span>(\r\n @Id var<\/span> id: String? = null<\/span>,\r\n var<\/span> initialBalance: Int? = 0<\/span>,\r\n var<\/span> remainingBalance: Int? =0<\/span>\r\n)\r\n<\/pre>\n2. Create the DataProvider<\/strong><\/h5>\n
private<\/span> final<\/span> CommandGateway commandGateway;<\/span>\r\nprivate<\/span> final<\/span> DataProvider<<\/span>CardSummary,<\/span> Void><\/span> dataProvider;<\/span>\r\n\r\npublic<\/span> GiftCardGUI<\/span>(<\/span>CommandGateway commandGateway,<\/span> QueryGateway queryGateway)<\/span> {<\/span>\r\n this<\/span>.<\/span>commandGateway<\/span> =<\/span> commandGateway;<\/span>\r\n this<\/span>.<\/span>dataProvider<\/span> =<\/span> dataProvider(<\/span>queryGateway);<\/span>\r\n}<\/span>\r\n<\/pre>\ndata class<\/span> DataQuery<\/span>(val<\/span> offset: Int, val<\/span> limit: Int)\r\nclass<\/span> SizeQuery<\/span>\r\n<\/pre>\n
\n<\/p>\nprivate<\/span> DataProvider<<\/span>CardSummary,<\/span> Void><\/span> dataProvider(<\/span>QueryGateway queryGateway)<\/span> {<\/span>\r\n return<\/span> new<\/span> AbstractBackEndDataProvider<<\/span>CardSummary,<\/span> Void>()<\/span> {<\/span>\r\n @Override<\/span>\r\n protected<\/span> Stream<<\/span>CardSummary><\/span> fetchFromBackEnd<\/span>(<\/span>Query<<\/span>CardSummary,<\/span> Void><\/span> query)<\/span> {<\/span>\r\n return<\/span> queryGateway.<\/span>query<\/span>(<\/span>new<\/span> DataQuery(<\/span>query.<\/span>getOffset<\/span>(),<\/span> query.<\/span>getLimit<\/span>()),<\/span>\r\n ResponseTypes.<\/span>multipleInstancesOf<\/span>(<\/span>CardSummary.<\/span>class<\/span>)<\/span>\r\n ).<\/span>join<\/span>()<\/span>\r\n .<\/span>stream<\/span>();<\/span>\r\n }<\/span>\r\n\r\n @Override<\/span>\r\n protected<\/span> int<\/span> sizeInBackEnd<\/span>(<\/span>Query<<\/span>CardSummary,<\/span> Void><\/span> query)<\/span> {<\/span>\r\n return<\/span> queryGateway.<\/span>query<\/span>(<\/span>new<\/span> SizeQuery(),<\/span>\r\n ResponseTypes.<\/span>instanceOf<\/span>(<\/span>Integer.<\/span>class<\/span>))<\/span>\r\n .<\/span>join<\/span>();<\/span>\r\n }<\/span>\r\n };<\/span>\r\n}<\/span>\r\n<\/pre>\n3. A Brief Review<\/strong><\/h5>\n