GraphQL:如何使用graphQL-java实现分页?

问题描述

目前，在 graphql-java 库.它确实具有一些基本的中继支持，在其中，我们可以创建connection，这是Facebook推荐的实现分页的方法.
此是有助于实现该目标的方法.但是，由于没有文档，因此很难理解此功能的工作原理.如果某人已经有一个允许使用graphql-java库进行基本查询(例如Add，delete，fetch等)的现有模型，那么有人可以细分他们添加分页支持的步骤吗?

Currently, I see no existing support for pagination in the graphql-java library. It does have some basic relay support, where-in, we can create a connection, Facebook's recommended way of implementing pagination.
This is the method which helps achieve that. However, with no documentation I'm finding it hard to understand how this function works. Can someone break-down the steps they would take to add pagination support if they already have an existing model which allows basic queries like Add, delete, fetch etc. using the graphql-java library?

推荐答案

您甚至不需要中继连接来支持分页.您的查询可以简单地接受页码和大小(或限制/偏移量)作为参数并返回列表-完成.但是，如果您想要中继连接，例如Book类型，您将执行以下操作:

You don't even need Relay connections to support pagination. Your query could simply accept a page number and size (or limit/offset) as arguments and return a list - done.But, if you wanted Relay connection for e.g. Book type, you'd do something like the following:

Relay relay = new Relay();
GraphQLOutputType book = ...; //build your normal Book object type
GraphQLObjectType bookEdge = relay.edgeType(book.getName(), book, null, Collections.emptyList());
GraphQLObjectType bookConnection = relay.connectionType(book.getName(), bookEdge, Collections.emptyList());

因此，您将拥有符合的BookConnection类型.中继连接规范.

As a result, you'd have a BookConnection type that conforms to the Relay connection spec.

对于具有基本GraphQL的示例，您有一个简单的Web应用程序此处.

As for the example with basic GraphQL, you have a simple web app here.

连接规范自然适合支持基于光标的分页的数据存储，但是在与不同的分页样式一起使用时需要一些创造力.

The connection spec naturally fits a data store that supports cursor based pagination, but needs some creativity when used with different pagination styles.

1)如果您希望使用基于简单偏移量的分页，则可以决定将after视为偏移量(表示将传递数字)，而将first视为限制:

1) If you wish to use simple offset based paging, you can decide to treat after as the offset (meaning a number would be passed), and first as the limit:

SELECT * FROM ORDER BY timestamp OFFSET $after LIMIT $first

before和last相同，只是方向不同.

The same for before and last, just different direction.

2)另一种方法是将after/before视为排序列的最后一次看到的值(这样，将传递实际的(混淆的)值):

2) Another way is to treat after/before as the last seen value of the sort column (so an actual (obfuscated) value would be passed):

SELECT * FROM ORDER BY timestamp WHERE timestamp > $after LIMIT $first

我还建议您看一下我的项目 graphql-spqr ，使用示例应用程序，使开发GraphQL API变得非常简单.

I'd also recommend you take a look at my project, graphql-spqr, with an example app, that makes developing GraphQL APIs dead simple.

例如，您将创建如下的分页结果:

For example, you'd create a paginated result like this:

public class BookService {
    @GraphQLQuery(name = "books")
    //make sure the argument names and types match the Relay spec
    public Page<Book> getBooks(@GraphQLArgument(name = "first") int first, @GraphQLArgument(name = "after") String after) {
        //if you decide to fetch from a SQL DB, you need the limit and offset instead of a cursor
        //so, you can treat "first" as count as "after" as offset
        int offset = Integer.valueOf(after);
        List<Book> books = getBooksFromDB(first, offset);
        Page<Book> bookPage = PageFactory.createOffsetBasedPage(books, totalBookCount, offset);
        return bookPage;
    }
}

还有许多其他方法可以创建Page实例，这只是最简单的方法.

There's many other ways to create a Page instance, this is just the most straight-forward one.

然后您将根据Java类生成模式:

You'd then generate a schema from your Java class:

GraphQLSchema schema = new GraphQLSchemaGenerator()
       .withOperationsFromSingleton(new BookService())
       .generate();
GraphQL graphQL = GraphQLRuntime.newGraphQL(schema).build();

并执行查询:

ExecutionResult result = graphQL.execute("{books(first:10, after:\"20\") {" +
                "   pageInfo {" +
                "       hasNextPage" +
                "   }," +
                "   edges {" +
                "       cursor, node {" +
                "           title" +
                "}}}}");

但是，再次，如果您不使用Relay，那么确实不需要使事情复杂化.如果您的存储设备自然支持基于光标的分页，请继续使用它.如果不是，则只需使用简单的limit/offset参数并返回一个列表，然后忘记连接规范.创建它是为了使Relay能够在各种情况下自动管理分页，因此，如果您不使用Relay和/或具有基于光标的分页的DB，那么它几乎总是完全过失.

But, again, if you are not using Relay there's really no need to overcomplicate things. If your storage supports cursor-based pagination naturally, go for it. If it doesn't, just use the simple limit/offset arguments and return a list, and forget the connection spec. It was created to enable Relay to automatically manage paging in various scenarios, so it's almost always a total overkill if you're not using Relay and/or a DB with cursor-based pagination.

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