Fields - add docs (#13)

* Create fields.md

* Update QueryInterpreter.php

docs/fields.md

@@ -0,0 +1,71 @@
+# Fields
+
+
+Look at the `HasFields` trait for all available types (string, int etc). 
+
+## Example
+
+```
+class DamageBuilder extends QueryBuilder
+{
+    public function fields(): array
+    {
+        return [
+            'id'            => $this->uuid('uuid'),
+            'name' => $this->string('name')->nullable(),
+            'created_at'      => $this->datetime('created_at'),
+            'updated_at'      => $this->datetime('updated_at'),
+        ];
+    }
+}
+```
+
+## Custom types
+
+Extend the QueryBuilder to modify or add specific types. Here we want the string to return an empty string `""`, instead of a `null` (for backwards compatiblity)
+
+```php
+abstract class QueryBuilder extends \Apitizer\QueryBuilder
+{
+    protected function string(string $key): Field
+    {
+        return $this->field($key, 'string')->transform(static function ($value) {
+            // Always cast to string, even if null, because of backwards compatibility
+            return (string) $value;
+        });
+    }
+}
+```
+
+
+## Callable fields 
+
+
+```php
+use Apitizer\Types\GeneratedField;
+
+class InvoiceBuilder {
+    public function fields(): array
+    {
+        return [
+            'total' => $this->generatedField('int', function ($row, GeneratedField $field) {
+                return 1;
+            });
+        ];
+    }
+}
+```
+
+Any `callable` is accepted, so something like this would also suffice:
+```php
+use Apitizer\Types\GeneratedField;
+
+class InvoiceBuilder {
+    public function fields(): array
+    {
+        return [
+            'total' => $this->generatedField('int', new CalculateOrderTotal),
+        ];
+    }
+}
+```

src/Apitizer/Interpreter/QueryInterpreter.php

@@ -4,7 +4,7 @@
 
 use Apitizer\Types\FetchSpec;
 use Apitizer\QueryBuilder;
-use Apitizer\Types\AbstractField;
+use Apitizer\Types\Field;
 use Apitizer\Types\Association;
 use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
@@ -42,7 +42,7 @@ private function applySelect(Builder $query, array $fields, array $additionalSel
         $selectKeys = array_merge([$query->getModel()->getKeyName()], $additionalSelects);
 
         foreach ($fields as $fieldOrAssoc) {
-            if ($fieldOrAssoc instanceof AbstractField) {
+            if ($fieldOrAssoc instanceof Field) {
                 // Also load any of the selected keys.
                 $selectKeys[] = $fieldOrAssoc->getKey();
             } else if ($fieldOrAssoc instanceof Association) {