Middleware functions.md

Using middleware functions on the requests

The params property in the endpoints section of the manifest lets you rename and map REST parameters corresponding to GraphQL operations arguments.

However, for some REST requests we might need to do a more complex parameter mapping that is hard to express in the manifest, or manipulate arguments on the request object before they are passed to the GraphQL's execute() function. One example is where our REST endpoints is designed to receive a simple query parameter, but the corresponding GraphQL operation expects that parameter to be wrapped within an object.

You can use a middleware function on the REST request to handle this situation.

First, create a middlewares.js file and call init() with the options argument specifying the pathname of that file in the middlewaresFile property of options:

const gql2restOptions  = {
	// ...
	middlewaresFile:  './middlewares.js', 
	// ...
};

//const expressRouter = GraphQL2REST.init(schema, execute, gql2restOptions);

Use path.resolve(__dirname, <PATH>) for relative paths.

Then add your middleware function to middlewares.js.

Middleware functions accept (req, route, verb, operation) and should return the Express request object modified by the function.

• req is the Express request object
• route is the REST endpoint route
• verb is the HTTP method
• operation is the GraphQL operation name

route, verb and operation are read-only and should not be modified.

Example:

/* middlewares.js */

const someMiddlewareFunc = (req, route, verb, operation) => {
    // ... insert your logic here 
    // possibly modify req object:
    req.body.newParam = { key: val }; 
    return req;
}

module.exports = { someMiddlewareFunc };

(The GraphQL operation will get argument newParam which is an object { key: val }.)

Next, edit the entry for the endpoint you want add middleware to, in the manifest file:

 "/users": {
 	"post": {
 		"operation": "createUser",
 		"requestMiddlewareFunction": "someMiddlewareFunc"
 	}
 },

When POST /users is invoked, function "someMiddlewareFunc" will be executed before passing the request to GraphQL server with the modified req object.

Next: the pre-processing step.

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