add isNull rule && readme

README.md

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+
+# Nova Nested Form
+
+This package allows you to include your nested relationships' forms into a parent form.
+
+# Installation
+
+```bash
+composer require yassi/nova-nested-form
+```
+
+# Update to 3.0
+
+The **afterFill** and **beforeFill** methods are no longer available.
+
+# Attach a new relationship form to a resource
+
+Simply add a NestedForm into your fields. The first parameter must be an existing NovaResource class and the second parameter (optional) must be an existing HasOneOrMany relationship in your model.
+
+```php
+namespace App\Nova;
+
+use Laravel\Nova\Fields\ID;
+use Illuminate\Http\Request;
+use Laravel\Nova\Fields\Text;
+use Laravel\Nova\Fields\Gravatar;
+use Laravel\Nova\Fields\Password;
+// Add use statement here.
+use Yassi\NestedForm\NestedForm;
+
+class User extends Resource
+{
+    ...
+    public function fields(Request $request)
+    {
+        return [
+            ID::make()->sortable(),
+
+            Gravatar::make(),
+
+            Text::make('Name')
+                ->sortable()
+                ->rules('required', 'max:255'),
+
+            Text::make('Email')
+                ->sortable()
+                ->rules('required', 'email', 'max:254')
+                ->creationRules('unique:users,email')
+                ->updateRules('unique:users,email,{{resourceId}}'),
+
+            Password::make('Password')
+                ->onlyOnForms()
+                ->creationRules('required', 'string', 'min:6')
+                ->updateRules('nullable', 'string', 'min:6'),
+
+            // Add NestedForm here.
+            NestedForm::make('Posts'),
+        ];
+    }
+```
+
+# Choose when to display the form
+
+For instance, if the nested form should only be available if the value of the "has_comments" attirbute is true, you can use:
+
+```php
+class Post extends Resource
+{
+    ...
+    public function fields(Request $request)
+    {
+        return [
+            Boolean::make('Has Comments'),
+            NestedForm::make('Comments')->displayIf(function ($nestedForm, $request) {
+               return [
+                    [ 'attribute' => 'has_comments', 'is' => true ]
+               ];
+        ];
+    }
+})
+```
+
+The **displayIf** method is excepted to return an array of array as you may want to add several conditions.
+
+```php
+class Post extends Resource
+{
+    ...
+    public function fields(Request $request)
+    {
+        return [
+            Boolean::make('Has Comments'),
+            Text::make('Title'),
+            Text::make('Subtitle')->nullable(),
+            Number::make('Number of comments allowed'),
+            NestedForm::make('Comments')->displayIf(function ($nestedForm, $request) {
+                return [
+                    [ 'attribute' => 'has_comments', 'is' => true ],
+                    [ 'attribute' => 'title', 'isNotNull' => true ],
+                    [ 'attribute' => 'subtitle', 'isNull' => true ],
+                    [ 'attribute' => 'title', 'includes' => 'My' ],
+                    [ 'attribute' => 'number_of_comments_allowed', 'moreThanOrEqual' => 1 ],
+                ];
+            })
+        ];
+    }
+}
+```
+
+The package will then add those conditions and dynamically update your form as you fill the fields. The available rules are:
+
+- [x] is
+- [x] isNot
+- [x] isNull
+- [x] isNotNull
+- [x] isMoreThan
+- [x] isMoreThanOrEqual
+- [x] isLessThan
+- [x] isLessThanOrEqual
+
+# Add a minimum or a maximum number of children
+
+For instance, if you want every user to have at least 3 posts and at most 5 posts, simply use:
+
+```php
+NestedForm::make('Posts')->min(3)->max(5),
+```
+
+Please note that the package automatically detects whether the relationship excepts many children or a single child, and sets the maximum value accordingly.
+
+When creating a new user, 3 blank posts will be displayed. If you reach the maximum number of posts, the "Add a new post" button will disappear.
+
+# Set the default open/collapse behavior
+
+If you want the nested forms to be opened by default, simply use:
+
+```php
+NestedForm::make('Posts')->open(true),
+```
+
+# Modify the default heading
+
+You can modify the default heading using the heading() method. You can use the helper method **wrapIndex()** to add the current child index to your header.
+
+```php
+NestedForm::make('Posts')->heading(NestedForm::wrapIndex() . ' // ' . Post'),
+```
+
+You can also add any attribute of the current child into your heading using the helper method **wrapAttribute()**.
+
+```php
+NestedForm::make('Posts')->heading(NestedForm::wrapIndex() . ' // ' . NestedForm::wrapAttribute('title', 'My default title')),
+```
+
+# Modify the index separator
+
+You can modify the default index separator using the separator() method when you have nested forms (e.g. 1. Post, 1.1. Comment, 1.1.1. Like).
+
+```php
+NestedForm::make('Posts')->separator('\'),
+```