Add validation rules docs

content/docs/db.json

@@ -35,6 +35,12 @@
     "contentPath": "./guides/seeders.md",
     "category": "Guides"
   },
+  {
+    "permalink": "validation",
+    "title": "Validation rules",
+    "contentPath": "./guides/validation.md",
+    "category": "Guides"
+  },
   {
     "permalink": "select-query-builder",
     "title": "Select query builder",

content/docs/guides/validation.md

@@ -0,0 +1,136 @@
+---
+summary: Validation rules added to VineJS through AdonisJS container
+---
+
+# Validation rules
+
+Lucid adds validation rules to VineJS to use in your schemas. Under the hood, it registers a provider in your AdonisJS application that extends VineJS rules.
+You can read more in the [AdonisJS docs](https://docs.adonisjs.com/guides/concepts/service-providers#service-providers) and [VineJS docs](https://vinejs.dev/docs/extend/custom_rules).
+
+You can use these rules directly from your VineJS schema. For example, the `unique` rule:
+
+```ts
+import vine from '@vinejs/vine'
+
+const schema = vine.object({
+  email: vine
+    .string()
+    // highlight-start
+    .unique({
+      table: 'users',
+      column: 'email',
+    }),
+  // highlight-end
+})
+```
+
+## Unique
+
+Ensure the value is unique (does not exists) inside a given database table and column.
+
+:::note
+The rule is a macro for `VineString` and `VineNumber`, so you can use it after `vine.string()` or `vine.number()`.
+:::
+
+You may either pass a callback to query the database directly, or an object:
+
+- The callback must return `true` if the value is unique (does not exist), or `false` if the value already exists.
+- You may also pass an options object to specify the table and column.
+
+```ts
+// Usage with callback
+const schema = vine.object({
+  email: vine
+    .string()
+    // highlight-start
+    .unique((db, value) => {
+      const row = await db.from('users').where('email', value).first()
+      return row === null
+    }),
+  // highlight-end
+})
+
+// Usage with options
+const schema = vine.object({
+  email: vine
+    .string()
+    // highlight-start
+    .unique({ table: 'users', column: 'email' }),
+  // highlight-end
+})
+```
+
+Following is the list of available parameters for the unique rule:
+
+```ts
+type UniqueRuleCallback = (db: Database, value: string, field: FieldContext) => Promise<boolean>
+
+type UniqueRuleOptions = {
+  table: string
+  column?: string
+  filter?: (db: DatabaseQueryBuilderContract, value: unknown, field: FieldContext) => Promise<void>
+}
+```
+
+You may also use your Lucid model directly inside the callback:
+
+```ts
+const schema = vine.object({
+  email: vine
+    .string()
+    // highlight-start
+    .unique((_, value) => {
+      const row = await User.findBy('email', value)
+      return row === null
+    }),
+  // highlight-end
+})
+```
+
+## Exists
+
+Ensure the value exists inside a given database table and column. This is the inverse of the unique rule.
+
+:::note
+The rule is also a macro for `VineString` and `VineNumber`, so you can use it after `vine.string()` or `vine.number()`.
+:::
+
+You may either pass a callback to query the database directly, or an object:
+
+- The callback must return `true` if the value exists, `false` otherwise.
+- You may also pass an option object to specify the table and column.
+
+```ts
+// Usage with callback
+const schema = vine.object({
+  slug: vine
+    .string()
+    // highlight-start
+    .exists((db, value) => {
+      const row = await db.from('categories').where('slug', value).first()
+      return row !== null
+    }),
+  // highlight-end
+})
+
+// Usage with options
+const schema = vine.object({
+  slug: vine
+    .string()
+    // highlight-start
+    .exists({ table: 'categories', column: 'slug' }),
+  // highlight-end
+})
+```
+
+Following is the list of available parameters for the exists rule:
+
+```ts
+type ExistsRuleCallback = (db: Database, value: string, field: FieldContext) => Promise<boolean>
+
+type ExistsRuleOptions = {
+  table: string
+  column?: string
+  filter?: (db: DatabaseQueryBuilderContract, value: unknown, field: FieldContext) => Promise<void>
+}
+```