chore: add context binding example

Author: davedx

README.md

@@ -608,6 +608,8 @@ const myLoader = new DataLoader(objResults(myBatchLoader))
 
 Looking to get started with a specific back-end? Try the [loaders in the examples directory](/examples).
 
+Want to access your request context from your DataLoader functions? There's an [example of how to do that too](/examples/ContextBinding.md)!
+
 ## Other Implementations
 
 Listed in alphabetical order

examples/ContextBinding.md

@@ -0,0 +1,40 @@
+# Binding context to your DataLoaders
+
+Sometimes in your DataLoader loading function you want to access a local context object of some sort, for example a [request context](https://www.apollographql.com/docs/apollo-server/data/resolvers/#the-context-argument) that could contain your database object, the current user, or some per-request telemetry. While not immediately obvious, it's simple to do this, and actually complements the best practice of creating a new DataLoader for each request. Here's how to do it (the example uses Apollo but this works with anything):
+
+```js
+function context(args: ExpressContext): Context {
+  const context = {
+    db: getDB(),
+    user: getUserFromSession(args)
+  };
+  context.loaders = {
+    comments: new DataLoader(commentDataloader.bind(context), {
+      context: context,
+    })
+  };
+  return context;
+}
+
+// Usage (`pg-promise` syntax)
+
+const resolvers = {
+  // ...
+  posts: {
+    comments: (parent: Post, args: Args, context: Context) => {
+      return context.loaders.comments.loadMany(parent.comments);
+    }
+  }
+};
+
+export async function commentDataloader(
+  this: Context,
+  parentIds: readonly number[]
+) {
+  // Now you can also do fine-grained authorization with `this.user`
+  return await this.db.manyOrNone(
+    "SELECT * from comment WHERE id IN ($[parentIds:list]) ORDER BY id DESC",
+    { parentIds }
+  );
+}
+```