chores(docs): added docs for using MongoDbOffsetDataSource

packages/mongodb/README.md

@@ -71,3 +71,108 @@ const booksOffsetPager = dataSourceLoaderPager({
 ```
 
 See fully working example in [examples/mongodb](../../examples/mongodb).
+
+## MongoDbOffsetDataSource Usage
+
+This module can be used  to perform both cursor based and offset based pagination on `MongoDbDatasources`.
+
+## Creating a paginated list of Books type
+
+- In this document we will discuss in more details about how to perform a cursor based pagination on a MongoDb datasource using the `MongoDbOffsetDataSource` module.
+- Steps to create a pager.
+    1. Create a mongoDb.
+
+        ```tsx
+        import { MongoClient } from 'mongodb';
+
+        const options = {} //set the required options
+        const mongoClient = new MongoClient("url to the mongoDb",{...options})
+        const mongoDb = mongoClient.db("dbName")
+        ```
+
+    2. Create a datasource using the `MongoDbOffsetDataSource` .
+
+        ```tsx
+        const ds = new MongoDbOffsetDataSource<BookType, QueryBooksArgs>({
+        	collectionName:"book",
+        	mongoDb,
+        	sortByFieldName: getSortByField,
+        	filters: getFilter
+        })
+        ```
+
+        While creating the “datasource” you can also provide two functions, namely `getSortByField` and `getFilter`.
+
+        ```tsx
+        // This function is used to tell that on what basis the documents must be 
+        // sorted and then execute the query. By default "_id" field is used.
+        const getSortByField = () => "Title"
+
+        // This function is used to return filters on the basis of which you 
+        // want to filter out the documents. Must return an array of valid 
+        // mongo db conditions.
+
+        // The getFilter function here will filter out the book which are written,
+        // in the year provided in args.
+        const getFilter = (args:QueryBooksArgs) => {
+        	const filter = [];
+        	if(args.publicYear === null) return filter
+        	filters.push({ publicYear : {$eq: args.publicYear} })
+        	return filter
+        }
+        ```
+
+        Using the combination of above two functions, we can easily fetch the books written in a particular year sorted alphabetically on the basis of Title. The order of sorting (ascending or descending) can be decided based on the use of `forwardResolver` or `backwardResolver` functions present on the pager objects (discussed later)
+
+    3. You should always wrap the object of type `MongoDbOffsetDataSource` in a `OffsetDataSourceWrapper` so that offset can be translated to cursor.
+
+        ```tsx
+        const dataSource = new OffsetDataSourceWrapper(ds);
+        ```
+
+    4. The next step would be to create a pager Object, while creating the pager object we also get access to the pager type available to be used while defining the schema, so that it can be used while defining the schema, in this case we will have the access to the `BookConnection` type.
+
+        ```tsx
+        const bookPager = dataSourcePager<Book, number, QueryBooksArgs>({
+        	typeName:"Book",
+        	fetchTotalCountInResolver: true,
+        	dataSource: datasource,
+        	typeDefDirectives: directiveOptions
+        })
+        ```
+
+        This pager object now can be used to perform both cursor based and page based pagination. 😉
+
+
+    We are now done with creating out pager object, next step would be to use this pager object to get paginated results 🎉.
+
+- Lets define a GraphQL query to fetch a list of Books. This Query will have an object of `BookConnection`  as return type.
+
+    ```tsx
+    //This query will return the list of book
+    type Query{
+    	books(
+    		first: Int! = 10
+    		page: Int
+    		after: String
+    		publicYear : Int
+    	): BookConnection!
+    }
+    ```
+
+- Lets write a resolver for this query.
+
+    ```tsx
+    // Here forward resolver can be used to get the list of books sorted in 
+    // ascending order by "Title",
+
+    export const Query: QueryResolvers<DataSourceContext> = {
+    	books:async (_,args,{bookPager}) => await bookPager.forwardResolver(args);
+    }
+
+    // To get the list in descending order we can do something like.
+    books: async (_,args,{bookPager}) => {
+    	const backwardArgs = { ...args, last: args.first, before: args.after };
+      return cvePager.backwardResolver(backwardArgs);
+    }
+    ```