Possibly exposing my ignorance of apollo-server but hoping someone can help: so ATM I have some schemas stitched together with #graphql-tools; all very simple, cool. I can make queries without problem.
There's a desire to add custom fields to given queries, so that we add extra data from other sources into the requested existing query template.
To explain by example: say the schema looks like this:
type User {
id
projectId
}
I'm trying to develop something so that the query getUserById($id...) can provide a template like so:
query userById($id: ID!) {
userById(id: $id) {
id
project {
id
name
# whatever other fields I want from Project type
}
}
}
And then apollo/graphql would then make a separate, asynchronous request to fetch the project for that given User.
As I understand graphql-tools, I can see resolvers allow the ability to make async requests for extra data ... but my problem is by defining project { within the query template, an error is thrown because - of course - project is not defined in the actual Schema itself.
Is there a way to filter and remove fields from a given query, somewhere in the chain of events? A custom apollo-server plugin perhaps? As I said I'm exposing my ignorance here but I've got a little lost in how apollo behaves in tandem with GraphQl.
Related
Im trying to learn GraphQL with Laravel & Lighthouse and have a question Im hoping someone can help me with. I have the following five database tables which are also defined in my Laravel models:
users
books
user_books
book_series
book_copies
I'd like to create a GraphQL endpoint that allows me to get back an array of users and the books they own, where I can pull data from multiple tables into one subfield called "books" like so:
query {
users {
name
books {
title
issue_number
condition
user_notes
}
}
}
To accomplish this in SQL is easy using joins like this:
$users = User::all();
foreach ($users as $user) {
$user['books'] = DB::select('SELECT
book_series.title,
book.issue_number
book_copies.condition,
user_books.notes as user_notes
FROM user_books
JOIN book_copies ON user_books.book_copy_id = book_copies.id
JOIN books ON book_copies.book_id = books.id
JOIN book_series ON books.series_id = book_series.id
WHERE user_books.user_id = ?',[$user['id']])->get();
}
How would I model this in my GraphQL schema file when the object type for "books" is a mashup of properties from four other object types (Book, UserBook, BookCopy, and BookSeries)?
Edit: I was able to get all the data I need by doing a query that looks like this:
users {
name
userBooks {
user_notes
bookCopy {
condition
book {
issue_number
series {
title
}
}
}
}
}
However, as you can see, the data is separated into multiple child objects and is not as ideal as getting it all in one flat "books" object. If anyone knows how I might accomplish getting all the data back in one flat object, Id love to know.
I also noticed that the field names for the relationships need to match up exactly with my controller method names within each model, which are camelCase as per Laravel naming conventions. Except for my other fields are matching the database column names which are lower_underscore. This is a slight nitpick.
Ok, after you edited your question, I will write the answer here, to answer your new questions.
However, as you can see, the data is separated into multiple child objects and is not as ideal as getting it all in one flat "books" object. If anyone knows how I might accomplish getting all the data back in one flat object, Id love to know.
The thing is, that this kind of fetching data is a central idea of GraphQL. You have some types, and these types may have some relations to each other. So you are able to fetch any relations of object, in any depth, even circular.
Lighthouse gives you out of the box support to eloquent relations with batch loading, avoiding the N+1 performance problem.
You also have to keep in mind - every field (literally, EVERY field) in your GraphQL definition is resolved on server. So there is a resolve function for each of the fields. So you are free to write your own resolver for particular fields.
You actually can define a type in your GraphQL, that fits your initial expectation. Then you can define a root Query field e.g. fetchUsers, and create you custom field resolver. You can read in the docs, how it works and how to implement this: https://lighthouse-php.com/5.2/the-basics/fields.html#hello-world
In this field resolver you are able to make your own data fetching, even without using any Laravel/Eloquent API. One thing you have to take care of - return a correct data type with the same structure as your return type in GraphQL for this field.
So to sum up - you have the option to do this. But in my opinion, you have to write more own code, cover it with tests on you own, which turns out in more work for you. I think it is simpler to use build-in directives, like #find, #paginate, #all in combination with relations-directives, which all covered with tests, and don't care about implementation.
I also noticed that the field names for the relationships need to match up exactly with my controller method names within each model, which are camelCase as per Laravel naming conventions.
You probably means methods within Model class, not controller.
Lighthouse provides a #rename directive, which you can use to define different name in GraphQL for your attributes. For the relation directives you can pass an relation parameter, which will be used to fetch the data. so for your example you can use something like this:
type User {
#...
user_books: [Book!]! #hasMany(relation: "userBooks")
}
But in our project we decided to use snak_case also for relations, to keep GraphQL clean with consistent naming convention and less effort
A neighborhood has many homes. Each home is owned by a person.
Say I have this graphql query:
{
neighborhoods {
homes {
owner {
name
}
}
}
}
I can preload the owners, and that'll make the data request be a single SQL query. Fine.
But what if I don't request the owner in the graphql query, the data will still be preloaded.
And if I don't preload, the data will either be fetched in every query, or not at all since I'm not loading the belongs_to association in the resolver.
I'm not sure if this is a solved issue, or just a painpoint one must swallow when working with graphql.
Using Absinthe, DataLoader and Elixir by the way.
Most GraphQL implementations, including Absinthe, expose some kind of "info" parameter that contains information specific to the field being resolved and the request being executed. You can parse this object to determine which fields were actually requested and build your SQL query appropriately.
See this issue for a more in-depth discussion.
In order to complement what Daniel Rearden said, you have to use the info.definition to resolve nested includes.
In my application I defined an array of possible values like:
defp relationships do
[
{:person, [tasks: [:items]]]}
...
]
end
then I have a logic that iterates over the info.definition and uses this function to preload the associations.
You will use a DataLoader to lazy load your resources. Usually to fetch third party requests or perform a complex database query.
What are all the different ways of updating the Apollo InMemoryCache after a mutation? From the docs, I can see:
Id-based updates which Apollo performs automatically
Happens for single updates to existing objects only.
Requires an id field which uniquely identifies each object, or the cache must be configured with a dataIdFromObject function which provides a unique identifier.
"Manual" cache updates via update functions
Required for object creation, deletion, or updates of multiple objects.
Involves calling cache.writeQuery with details including which query should be affected and how the cache should be changed.
Passing the refetchQueries option to the useMutation hook
The calling code says which queries should be re-fetched from the API, Apollo does the fetching, and the results replace whatever is in the cache for the given queries.
Are there other ways that I've missed, or have I misunderstood anything about the above methods?
I am confused because I've been reading the code of a project which uses Apollo for all kinds of mutations, including creations and deletions, but I don't see any calls to cache.writeQuery, nor any usage of refetchQueries. How does the cache get updated after creations and deletions without either of those?
In my own limited experience with Apollo, the cache is not automatically updated after an object creation or deletion, not even if I define dataIdFromObject. I have to update the cache myself by writing update functions.
So I'm wondering if there is some secret config I've missed to make Apollo handle it for me.
The only way to create or delete a node and have Apollo automatically update the cache to reflect the change is to return the parent field of whatever field contains the updated List field. For example, let's say we have a schema like this:
type Query {
me: User
}
type User {
id: ID!
posts: [Post!]!
}
type Post {
id: ID!
body: String!
}
By convention, if we had a mutation to add a new post, the mutation field would return the created post.
type Mutation {
writePost(body: String!): Post!
}
However, we could have it return the logged in User instead (the same thing the me field returns):
type Mutation {
writePost(body: String!): User!
}
by doing so, we enable the client to make a query like:
mutation WritePost($body: String!){
writePost(body: $body) {
id
posts {
id
body
}
}
}
Here Apollo will not only create or update the cache for all the returned posts, but it will also update the returned User object, including the list of posts.
So why is this not commonly done? Why does Apollo's documentation suggest using writeQuery when adding or deleting nodes?
The above will work fine when your schema is simple and you're working with a relatively small amount of data. However, returning the entire parent node, including all its relations, can be noticeably slower and more resource-intensive once you're dealing with more data. Additionally, in many apps a single mutation could impact multiple queries inside the cache. The same node could be returned by any number of fields in the schema, and even the same field could be part of a number of different queries that utilize different filters, sort parameters, etc.
These factors make it unlikely that you'll want to implement this pattern in production but there certainly are use cases where it may be a valid option.
I was reading this article and it used the following query:
{
getAuthor(id: 5){
name
posts {
title
author {
name # this will be the same as the name above
}
}
}
}
Which was parsed and turned into an AST like the one below:
Clearly it is bringing back redundant information (the Author's name is asked for twice), so I was wondering how GraphQL Handles that. Does it redundantly fetch that information? Is the diagram a proper depiction of the actual AST?
Any insight into the query parsing and execution process relevant to this would be appreciated, thanks.
Edit: I know this may vary depending on the actual implementation of the GraphQl server, but I was wondering what the standard / best practice was.
Yes, GraphQL may fetch the same information multiple times in this scenario. GraphQL does not memoize the resolver function, so even if it is called with the same arguments and the same parent value, it will still run again.
This is a fairly common problem when working with databases in GraphQL. The most common solution is to utilize DataLoader, which not only batches your database requests, but also provides a cache for those requests for the duration of the GraphQL request. This way, even if a particular record is requested multiple times, it will only be fetched from the database once.
The alternative (albeit more complicated) approach is to compose a single database query based on the requested fields that executes at the root level. For example, our resolver for getAuthor could constructor a single query that would return the author, their posts and each of that post's author. With this approach, we can skip writing resolvers for the posts field on the Author type or the author field on the Post type and just utilize the default resolver behavior. However, in order to do this and avoid overfetching, we have to parse the GraphQL request inside the getAuthor resolver in order to determine which fields were requested and should therefore be included in our database query.
In writing this question I realised that there is something very specific I want to be able to do in GraphQL, and I can't see a good way of implementing it. The idea is this:
One of the nice things about GraphQL is that it allows you to make flexible queries. For example, if I want to find all the comments on all the posts of each user in a particular forum then I can make the query
query{
findForum(id:7){
users{
posts{
comments{
content
}
}
}
}
}
which is great. Often, you want to collect data with the intention of mutating it. So in this case, maybe I don't want to fetch all of those comments, and instead I want to delete them. A naive suggestion is to implement a deleteComment field on the comment type, which mutates the object it is called on. This is bad because the request is tagged as a query, so it should not mutate data.
Since we're mutating data, we should definitely tag this as a mutation. But then we lose the ability to make the query we wanted to make, because findForum is a query field, not a mutation field. A way around this might be to redefine all the query fields you need inside the mutation type. This is obviously not a good idea, because you repeat a lot of code, and also make the functionality for query a strict subset of that of mutation.
Now, what I regard as the 'conventional' solution is to make a mutation field which does this job and nothing else. So you define a mutation field deleteAllUserPostCommentsByForum which takes an argument, and implement it in the obvious way. But now you've lost the flexibility! If you decide instead that you want to find the user explicitly, and delete all their posts, or if you only want to delete some of their posts, you need a whole new mutation field. This feels like precisely the sort of thing I though GraphQL was useful for when compared to REST.
So, is there a good way to avoid these problems simultaneously?
Under the hood, the only real difference between queries and mutations is that if a single operation includes multiple mutations, they are resolved sequentially (one at a time) rather than concurrently. Queries, and all other fields are resolved concurrently. That means for an operation like this:
mutation myOperation {
editComment(id: 1, body: "Hello!")
deleteComment(id: 1)
}
The editComment mutation will resolve before the deleteComment mutation. If these operations were queries, they would both be ran at the same time. Likewise, consider if you have a mutation that returns an object, like this:
mutation myOperation {
deleteComment(id: 1) {
id
name
}
}
In this case, the id and name fields are also resolved at the same time (because, even though they are returned as part of a mutation, the fields themselves are not mutations).
This difference in behavior between queries and mutations highlights why by convention we define a single mutation per operation and avoid "nesting" mutations like your question suggests.
The key to making your mutations more flexible lies in how you pass in inputs to your mutation subsequently how you handle those inputs inside your resolver. Instead of making a deleteAllUserPostCommentsByForum mutation, just make a deleteComments mutation that accepts a more robust InputType, for example:
input DeleteCommentsInput {
forumId: ID
userId: ID
}
Your resolver then just needs to handle whatever combination of input fields that may be passed in. If you're using a db, this sort of input very easily translates to a WHERE clause. If you realize you need additional functionality, for example deleting comments before or after a certain date, you can then add those fields to your Input Type and modify your resolver accordingly -- no need to create a new mutation.
You can actually handle creates and edits similarly and keep things a little DRY-er. For example, your schema could look like this:
type Mutation {
createOrUpdateComment(comment: CommentInput)
}
input CommentInput {
id: ID
userId: ID
body: String
}
Your resolver can then check whether an ID was included -- if so, then it treats the operation as an update, otherwise it treats the operation as an insert. Of course, using non-nulls in this case can get tricky (userId might be needed for a create but not an update) so there's something to be said for having separate Input Types for each kind of operation. However, hopefully this still illustrates how you can leverage input types to make your mutations more flexible.
IMHO you lose many indirect aspects.
Trying to create 'flexible' query can result in highly unoptimized server actions.
Queries are resolved structurally, level by level, which may result in processing to many unnecessary data (high memory usage). It can't be optimized on lower layers (f.e. sql server) - it will result in a naive implementation (processing) like many 'manually fired' SQL queries vs. one more complex query with conditions.
In this case f.e. server doesn't need all users at all while user's post/comment usually contain user_id (and forum/thread/post ids) field - it can be processed directly on one table (with joined posts). You don't need the whole structure to affect only some of the elements.
The real power and flexibility of graphQL are placed on the resolvers.
Notice that deleting all or only some comments can be completely different implemented. Resolver can choose a better way (by parameters as Daniel wrote) but for simplicity (readability of the API) it would be better to have a separate mutations.