[Feature] Allow non-camel case adapter relation method names.

Although developers should follow the PSR1 standard of camel
casing method names, there are some developers who follow
the Eloquent attribute standard of snake casing attributes
when defining relationships.

This commit allows an adapter to have its relation method name
in exactly the same format as the JSON API field name. I.e.
a field name of `user_history` and a relation method called
`user_history`.

Otherwise it falls back to the PSR1 standard and expects the
method name to be the camel case version of the field name.

See #315 for discussion.

Author: lindyhopchris

CHANGELOG.md

@@ -2,6 +2,14 @@
 All notable changes to this project will be documented in this file. This project adheres to
 [Semantic Versioning](http://semver.org/) and [this changelog format](http://keepachangelog.com/).
 
+## Unreleased
+
+### Added
+- [#315](https://github.com/cloudcreativity/laravel-json-api/issues/315)
+Allow developers to use the exact JSON API field name as the relationship method name on their
+adapters. Although we recommend following the PSR1 standard of using camel case for method names,
+this does allow a developer to use snake case field names with snake case method names.
+
 ## [1.0.1] - 2019-03-12
 
 ### Fixed

src/Adapter/AbstractResourceAdapter.php

@@ -197,11 +197,32 @@ protected function isFillableRelation($field, $record)
     }
 
     /**
-     * @param $field
+     * Get the method name on this adapter for the supplied JSON API field.
+     *
+     * By default we expect the developer to be following the PSR1 standard,
+     * so the method name on the adapter should use camel case.
+     *
+     * However, some developers may prefer to use the actual JSON API field
+     * name. E.g. they could use `user_history` as the JSON API field name
+     * and the method name.
+     *
+     * Therefore we return the field name if it exactly exists on the adapter,
+     * otherwise we camelize it.
+     *
+     * A developer can use completely different logic by overloading this
+     * method.
+     *
+     * @param string $field
+     *      the JSON API field name.
      * @return string|null
+     *      the adapter's method name, or null if none is implemented.
      */
     protected function methodForRelation($field)
     {
+        if (method_exists($this, $field)) {
+            return $field;
+        }
+
         $method = Str::camelize($field);
 
         return method_exists($this, $method) ? $method : null;