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25.

API Pagination Done Easily

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Let's do some pagination! Technically, HAL doesn't have any opinion on how all of this should work. But fortunately, the rest of the REST world does. As you can see in this example, a great way to handle pagination is with links (from http://phlyrestfully.readthedocs.org/en/latest/halprimer.html#collections):

{
    "_links": {
        "self": {
            "href": "http://example.org/api/user?page=3"
        },
        "first": {
            "href": "http://example.org/api/user"
        },
        "prev": {
            "href": "http://example.org/api/user?page=2"
        },
        "next": {
            "href": "http://example.org/api/user?page=4"
        },
        "last": {
            "href": "http://example.org/api/user?page=133"
        }
    }
    "count": 3,
    "total": 498,
    "_embedded": {
        "users": [
            {
                "_links": {
                    "self": {
                        "href": "http://example.org/api/user/mwop"
                    }
                },
                "id": "mwop",
                "name": "Matthew Weier O'Phinney"
            },
            {
                "_links": {
                    "self": {
                        "href": "http://example.org/api/user/mac_nibblet"
                    }
                },
                "id": "mac_nibblet",
                "name": "Antoine Hedgecock"
            },
            {
                "_links": {
                    "self": {
                        "href": "http://example.org/api/user/spiffyjr"
                    }
                },
                "id": "spiffyjr",
                "name": "Kyle Spraggs"
            }
        ]
    }
}

The keys here - first, prev, next and last - aren't accidental. These are Internet-wide standards like self. So if you use these names for your links, your API will be consistent with a lot of other APIs.

Pagination: Do it with Query Parameters

The other important thing to notice in this example is that pagination is done with query parameters. Technically, there are a lot of ways the client could tell us what page they want for a collection, like query parameters or request headers. But honestly, query parameters are the easiest way. In our case, we're going to follow what you see here exactly. And with the HATEOAS library, this will be easy.

Pagination Scenario

First, let's setup a scenario to test this in programmers.feature. In this scenario, we're going to do something really cool: we're going to follow the links for pagination. I want to be able to go to our collection resource, get the URL for this next link and make a second request to the next link and actually see what's on page 2.

For our pagination, we'll show 5 programmers per page. In the Given, we need to add a bunch of programmers to try this out - I'll paste in some very imaginative code that gives us 12 programmers in the database:

166 lines | features/api/programmer.feature
// ... lines 1 - 93
# we will do 5 per page
Scenario: Paginate through the collection of programmers
Given the following programmers exist:
| nickname |
| Programmer1 |
| Programmer2 |
| Programmer3 |
| Programmer4 |
| Programmer5 |
| Programmer6 |
| Programmer7 |
| Programmer8 |
| Programmer9 |
| Programmer10 |
| Programmer11 |
| Programmer12 |
// ... lines 110 - 166

Everything after this will be very similar to the normal collection resource, so I'll grab a the second half of that scenario. I'll remove the status code 200, because we're already testing for this above. After I make the first GET request, we're going to parse through the response, find the next link and make a second GET request. I already have a built-in step definition to do exactly that. I'll just say: And I follow the "next" link:

166 lines | features/api/programmer.feature
// ... lines 1 - 94
Scenario: Paginate through the collection of programmers
Given the following programmers exist:
// ... lines 97 - 109
When I request "GET /api/programmers"
And I follow the "next" link
// ... lines 112 - 166

If you looked at the implementation behind this - which I wrote - it knows that we're using HAL, so it knows to look at _links, next, href and make a second GET request for that.

So on the second page, we'd expect there to be Programmer7, so we can say: And the "_embedded.programmers" property should contain "Programmer7", because we expect that word to be somewhere inside that JSON. And we expect there to not be Programmer2 and for there to also not be Programmer11:

166 lines | features/api/programmer.feature
// ... lines 1 - 93
# we will do 5 per page
Scenario: Paginate through the collection of programmers
// ... lines 96 - 111
Then the "_embedded.programmers" property should contain "Programmer7"
But the "_embedded.programmers" property should not contain "Programmer2"
But the "_embedded.programmers" property should not contain "Programmer11"
// ... lines 115 - 166

Programmer2 should be on page 1 and Programmer11 should be on page 3.

This starts on line 96, so let's try the scenario out:

php vendor/bin/behat features/api.programmer.feature:96

It fails of course because it can't find the next link - our collection response doesn't have that because we haven't done any of the work for it yet.

If we look at the HATEOAS documentation, they talk about pagination. To do pagination, you'll return this PaginatedRepresentation resource. So let's create that in ProgrammerController. So here is listAction where we're getting our programmers. And remember, right now we're creating a CollectionResource, and that's what you return when you have a collection resource, but you don't need it to have pagination:

192 lines | src/KnpU/CodeBattle/Controller/Api/ProgrammerController.php
// ... lines 1 - 88
public function listAction()
{
$programmers = $this->getProgrammerRepository()->findAll();
$collection = new CollectionRepresentation(
$programmers,
'programmers',
'programmers'
);
// ... lines 98 - 101
}
// ... lines 103 - 192

Below this, we'll say $paginated = new PaginatedRepresentation. My IDE just added the use statement at the top for us - make sure you have it:

205 lines | src/KnpU/CodeBattle/Controller/Api/ProgrammerController.php
// ... lines 1 - 5
use Hateoas\Representation\PaginatedRepresentation;
// ... lines 7 - 89
public function listAction()
{
// ... lines 92 - 97
$collection = new CollectionRepresentation(
$programmers,
'programmers',
'programmers'
);
$paginated = new PaginatedRepresentation(
// ... lines 104 - 109
);
// ... lines 111 - 114
}
// ... lines 116 - 205

This takes a number of different arguments. The first is the actual CollectionRepresentation. The second is the route name to the list endpoint, which for us is api_programmers_list. And it'll use this to generate the links like next, first and last. The third argument is any array of parameters that need to be passed to the route. So if the route had a nickname or id wildcard, you'd pass that here. But there aren't any wildcards in this route, so we'll pass an empty array. The next three arguments are the page we're on, the number of records we're showing per page, and the total number of pages. I'm going to invent a few variables and set them above in a moment:

205 lines | src/KnpU/CodeBattle/Controller/Api/ProgrammerController.php
// ... lines 1 - 89
public function listAction()
{
// ... lines 92 - 102
$paginated = new PaginatedRepresentation(
$collection,
'api_programmers_list',
array(),
$page,
$limit,
$numberOfPages
);
// ... lines 111 - 114
}
// ... lines 116 - 205

Above, let's fill this in. Initially, I just want to get the next and last _links to show up, so I'm going to take some shortcuts and not worry about truly paginating the results quite yet. I'll hardcode these variables for now. I'll say that we're always on page 1, that the limit is always 5, and we can calculate the total number of pages. If we have 12 programmers, divided by 5 gives us 2.4, then we'll round that up with the ceil function:

205 lines | src/KnpU/CodeBattle/Controller/Api/ProgrammerController.php
// ... lines 1 - 89
public function listAction()
{
// ... lines 92 - 93
$limit = 5;
$page = 1;
$numberOfPages = (int) ceil(count($programmers) / $limit);
// ... lines 97 - 102
$paginated = new PaginatedRepresentation(
$collection,
'api_programmers_list',
array(),
$page,
$limit,
$numberOfPages
);
// ... lines 111 - 113
return $response;
// ... lines 115 - 205

Normally you'd use a library to help with pagination, but since I'm faking it, I'm just doing some manual work myself.

Finally, now that we have this $paginated object, we'll pass it to createApiResponse:

205 lines | src/KnpU/CodeBattle/Controller/Api/ProgrammerController.php
// ... lines 1 - 89
public function listAction()
{
// ... lines 92 - 102
$paginated = new PaginatedRepresentation(
$collection,
'api_programmers_list',
array(),
$page,
$limit,
$numberOfPages
);
$response = $this->createApiResponse($paginated, 200, 'json');
return $response;
}
// ... lines 116 - 205

Cool. We'll still returning all of the programmers, so I don't expect our test to pass, but let's try it:

php vendor/bin/behat features/api.programmer.feature:96

Our test fails, but it is getting further. Ah, it actually is following the next link and it is actually seeing that the Programmer7 resource is in the response. But then it's failing because Programmer2 and every other programmer is still there.

What's really cool is that if we go back to our Hal browser and go to /api/programmers, those links are showing up. We also have some nice properties that tell us about the collection. And we can start paginating through the resources by following those links. And the way the links are done is just via ?page=1, ?page=2&limit=5.

Adding Real Pagination

Let's turn this into real pagination! Since the page and limit are being passed as query parameters, let's use those. In my application, when I need request information, I just add a Request $request argument to my controller:

208 lines | src/KnpU/CodeBattle/Controller/Api/ProgrammerController.php
// ... lines 1 - 89
public function listAction(Request $request)
{
// ... lines 92 - 117
}
// ... lines 119 - 208

Then for query parameters, I can say $request->query->get('page') and the second argument is the default value if there is no page sent for some reasons. And the same for limit - we'll let the client control this, but we'll default to 5:

208 lines | src/KnpU/CodeBattle/Controller/Api/ProgrammerController.php
// ... lines 1 - 89
public function listAction(Request $request)
{
// ... lines 92 - 93
$limit = $request->query->get('limit', 5);
$page = $request->query->get('page', 1);
// ... lines 96 - 117
}
// ... lines 119 - 208

Normally when you do pagination, you might do a LIMIT, OFFSET query to the database or pass which subset of records you want to some search engine, like Elastic Search. In both cases, we end up with just the 5 records we want, instead of all 10,000. Here, I'm going to be lazy and not do that. I'm just going to query for all of my programmers, then just use a little PHP array magic to only give us the ones we want. I'm just trying to keep things simple.

If I were doing this in a real project, I'd probably use a library called Pagerfanta. It helps you paginate and has built-in adapters already for Doctrine, and Elastic Search. You can give it things like, we're on page 2, we're showing 5 results per page, and then it'll do the work to find only the results we need.

Instead of that, I'm going to paste in some manual logic and use array_slice:

208 lines | src/KnpU/CodeBattle/Controller/Api/ProgrammerController.php
// ... lines 1 - 89
public function listAction(Request $request)
{
// ... lines 92 - 93
$limit = $request->query->get('limit', 5);
$page = $request->query->get('page', 1);
// my manual, silly pagination logic. Use a real library
$offset = ($page - 1) * $limit;
$numberOfPages = (int) ceil(count($programmers) / $limit);
$collection = new CollectionRepresentation(
// my manual, silly pagination logic. Use a real library
array_slice($programmers, $offset, $limit),
// ... lines 103 - 104
);
// ... lines 106 - 117
}
// ... lines 119 - 208

So I'm querying for all of the programmers and then slicing those down to the ones I want. It's not an efficient way to do this, but we should have a functional, paginated endpoint now. So let's try the test again, and it works!

php vendor/bin/behat features/api.programmer.feature:96

Yes!

And to enjoy it a little bit more, we can go back to the Hal Browser, hit Go, and since we're on page 2, we see Programmers 6-10. We can follow links to the first page, then over to the last page to see only those 2 programmers. Now pagination isn't hard, and it'll be really consistent across your API.