Adds Graphiti example to docs

Author: NeedleInAJayStack

README.md

@@ -6,27 +6,34 @@ This is a Swift version of the Facebook [DataLoader](https://github.com/facebook
 [![Swift][swift-badge]][swift-url]
 [![License][mit-badge]][mit-url]
 
-## Installation 💻
+## Gettings started 🚀
 
 Include this repo in your `Package.swift` file.
 
 ```swift
 .Package(url: "https://github.com/GraphQLSwift/DataLoader.git", .upToNextMajor(from: "1.1.0"))
 ```
 
-## Gettings started 🚀
-### Batching
+To get started, create a DataLoader. Each DataLoader instance represents a unique cache. Typically instances are created per request when used
+within a web-server if different users can see different things.
+
+## Batching
 Batching is not an advanced feature, it's DataLoader's primary feature.
 
 Create a DataLoader by providing a batch loading function:
 ```swift
 let userLoader = Dataloader<Int, User>(batchLoadFunction: { keys in
   try User.query(on: req).filter(\User.id ~~ keys).all().map { users in
-    return users.map { DataLoaderFutureValue.success($0) }
+    keys.map { key in
+      DataLoaderFutureValue.success(users.filter{ $0.id == key })
+    }
   }
 })
 ```
-#### Load individual keys
+
+The order of the returned DataLoaderFutureValues must match the order of the keys.
+
+### Load individual keys
 ```swift
 let future1 = try userLoader.load(key: 1, on: eventLoopGroup)
 let future2 = try userLoader.load(key: 2, on: eventLoopGroup)
@@ -35,13 +42,13 @@ let future3 = try userLoader.load(key: 1, on: eventLoopGroup)
 
 The example above will only fetch two users, because the user with key `1` is present twice in the list. 
 
-#### Load multiple keys
+### Load multiple keys
 There is also a method to load multiple keys at once
 ```swift
 try userLoader.loadMany(keys: [1, 2, 3], on: eventLoopGroup)
 ```
 
-#### Execution
+### Execution
 By default, a DataLoader will wait for a short time (2ms) from the moment `load` is called to collect keys prior
 to running the `batchLoadFunction` and completing the `load` futures. This is to let keys accumulate and 
 batch into a smaller number of total requests. This amount of time is configurable using the `executionPeriod` 
@@ -65,12 +72,12 @@ If desired, you can manually execute the `batchLoadFunction` and complete the fu
 Scheduled execution can be disabled by setting `executionPeriod` to `nil`, but be careful - you *must* call `.execute()` 
 manually in this case. Otherwise, the futures will never complete.
 
-#### Disable batching
+### Disable batching
 It is possible to disable batching by setting the   `batchingEnabled` option to `false`
 It will invoke the `batchLoadFunction` immediately when a key is loaded.
 
 
-### Caching
+## Caching
 
 DataLoader provides a memoization cache. After `.load()` is called with a key, the resulting value is cached 
 for the lifetime of the DataLoader object. This eliminates redundant loads.
@@ -85,7 +92,7 @@ let future2 = userLoader.load(key: 1, on: eventLoopGroup)
 assert(future1 === future2)
 ```
 
-#### Caching per-Request
+### Caching per-Request
 
 DataLoader caching *does not* replace Redis, Memcache, or any other shared
 application-level cache. DataLoader is first and foremost a data loading mechanism,
@@ -98,7 +105,7 @@ could result in cached data incorrectly appearing in each request. Typically,
 DataLoader instances are created when a Request begins, and are not used once the
 Request ends.
 
-#### Clearing Cache
+### Clearing Cache
 
 In certain uncommon cases, clearing the request cache may be necessary.
 
@@ -124,7 +131,7 @@ userLoader.load(key: 4, on: eventLoopGroup)
 // Request completes.
 ```
 
-#### Caching Errors
+### Caching Errors
 
 If a batch load fails (that is, a batch function throws or returns a DataLoaderFutureValue.failure(Error)), 
 then the requested values will not be cached. However if a batch
@@ -142,7 +149,7 @@ userLoader.load(key: 1, on: eventLoopGroup).catch { error in {
 }
 ```
 
-#### Disabling Cache
+### Disabling Cache
 
 In certain uncommon cases, a DataLoader which *does not* cache may be desirable.
 Calling `DataLoader(options: DataLoaderOptions(cachingEnabled: false), batchLoadFunction: batchLoadFunction)` will ensure that every
@@ -184,6 +191,94 @@ let myLoader = DataLoader<String, String>(batchLoadFunction: { keys in
 })
 ```
 
+## Using with GraphQL
+
+DataLoader pairs nicely well with [GraphQL](https://github.com/GraphQLSwift/GraphQL) and
+[Graphiti](https://github.com/GraphQLSwift/Graphiti). GraphQL fields are designed to be 
+stand-alone functions. Without a caching or batching mechanism,
+it's easy for a naive GraphQL server to issue new database requests each time a
+field is resolved.
+
+Consider the following GraphQL request:
+
+```
+{
+  me {
+    name
+    bestFriend {
+      name
+    }
+    friends(first: 5) {
+      name
+      bestFriend {
+        name
+      }
+    }
+  }
+}
+```
+
+Naively, if `me`, `bestFriend` and `friends` each need to request the backend,
+there could be at most 13 database requests!
+
+By using DataLoader, we could batch our requests to a `User` type, and 
+only require at most 4 database requests, and possibly fewer if there are cache hits.
+Here's a full example using Graphiti:
+
+```swift
+struct User : Codable {
+    let id: Int
+    let name: String
+    let bestFriendID: Int
+    let friendIDs: [Int]
+    
+    func getBestFriend(context: UserContext, arguments: NoArguments, group: EventLoopGroup) throws -> EventLoopFuture<User> {
+        return try context.userLoader.load(key: user.bestFriendID, on: group)
+    }
+    
+    struct FriendArguments {
+        first: Int
+    }
+    func getFriends(context: UserContext, arguments: FriendArguments, group: EventLoopGroup) throws -> EventLoopFuture<[User]> {
+        return try context.userLoader.loadMany(keys: user.friendIDs[0..<arguments.first], on: group)
+    }
+}
+
+struct UserResolver {
+    public func me(context: UserContext, arguments: NoArguments) -> User {
+        ...
+    }
+}
+
+class UserContext {
+    let database = ...
+    let userLoader = DataLoader<Int, User>() { keys in
+        return User.query(on: database).filter(\.$id ~~ keys).all().map { users in
+            keys.map { key in
+                users.first { $0.id == key }!
+            }
+        }
+    }
+}
+
+struct UserAPI : API {
+    let resolver = UserResolver()
+    let schema = Schema<UserResolver, UserContext> {
+        Type(User.self) {
+            Field("name", at: \.content)
+            Field("bestFriend", at: \.getBestFriend, as: TypeReference<User>.self)
+            Field("friends", at: \.getFriends, as: [TypeReference<User>]?.self) {
+                Argument("first", at: .\first)
+            }
+        }
+        
+        Query {
+            Field("me", at: UserResolver.hero, as: User.self)
+        }
+    }
+}
+```
+
 ## Contributing 🤘
 
 All your feedback and help to improve this project is very welcome. Please create issues for your bugs, ideas and
@@ -201,6 +296,8 @@ This library is entirely a Swift version of Facebooks [DataLoader](https://githu
 Developed by  [Lee Byron](https://github.com/leebyron) and [Nicholas Schrock](https://github.com/schrockn) 
 from [Facebook](https://www.facebook.com/).
 
+
+
 [swift-badge]: https://img.shields.io/badge/Swift-5.2-orange.svg?style=flat
 [swift-url]: https://swift.org