Override One Array Element

+ and replace treat an array as a whole. Sometimes you need to reach INTO one existing element and add fields to it. The real case: the Creative Library's Items.json has a Children array of categories, and you want to add SubCategories to the stock Tools category without disturbing the other six categories.

Goal

• Add a SubCategories list to the existing { "Id": "Tools" } element.
• Leave Weapons, Armors, Foods, Potions, Recipes, Ingredients and the Tools element's own Name/Icon exactly as they are.

The Asset

Server/Item/Category/CreativeLibrary/Items.json

Without an element-targeting operator your only options are replace (restate every element) or append (a new sibling category in its own tab, not under Tools).

{
  "Icon": "Icons/ItemCategories/Items.png",
  "Order": 2,
  "Children": [
    {
      "Id": "Tools",                                  // the ONE element you want to extend
      "Name": "server.ui.itemcategory.tools",
      "Icon": "Icons/ItemCategories/Items-Tools.png"
    },
    {
      "Id": "Weapons",
      "Name": "server.ui.itemcategory.weapons",
      "Icon": "Icons/ItemCategories/Items-Weapons.png"
    },
    {
      "Id": "Armors",
      "Name": "server.ui.itemcategory.armors",
      "Icon": "Icons/ItemCategories/Items-Armor.png"
    },
    {
      "Id": "Foods",
      "Name": "server.ui.itemcategory.foods",
      "Icon": "Icons/ItemCategories/Items-Food.png"
    },
    {
      "Id": "Potions",
      "Name": "server.ui.itemcategory.potions",
      "Icon": "Icons/ItemCategories/Items-Potion.png"
    },
    {
      "Id": "Recipes",
      "Name": "server.ui.itemcategory.recipes",
      "Icon": "Icons/ItemCategories/Items-Recipe.png"
    },
    {
      "Id": "Ingredients",
      "Name": "server.ui.itemcategory.ingredients",
      "Icon": "Icons/ItemCategories/Items-Ingredients.png"
    }
  ]
}

The patch

Path: Server/Item/Category/CreativeLibrary/Items.patch

{
  "Children+": [
    {
      "$Match": "Id",
      "Id": "Tools",
      "SubCategories+": [
        { "Id": "Staffs", "Name": "server.ui.itemcategory.subcategory.staffs.name", "Order": 100 }
      ]
    }
  ]
}

Children~ opts the array into element merging. $Match: "Id" finds the element whose Id is Tools. The rest of the object merges into it, and the nested SubCategories+ appends (creating the array, since Tools had none).

After

• The Tools element now has a SubCategories array with your Staffs entry.
• Tools keeps its Name and Icon. Every sibling category is untouched.
• $Match is stripped and never appears in the merged asset.

{
  "Id": "Tools",
  "Name": "server.ui.itemcategory.tools",
  "Icon": "Icons/ItemCategories/Items-Tools.png",
  "SubCategories": [
    { "Id": "Staffs", "Name": "server.ui.itemcategory.subcategory.staffs.name", "Order": 100 }
  ]
}

Targeting by position instead

Not every array is keyed by a field. When there is nothing to match on, drop $Match and let ~ align by position. The patch element at each index merges into the base element at the same index, and an empty {} merges nothing, so it skips a slot:

{ "SomeList~": [ {}, { "Enabled": false } ] }

This leaves index 0 alone and sets Enabled on index 1.

Notes

• ~ is the element-merge operator; $Match is an optional locator inside it (and inside +). There is no implicit Id matching: matching happens only where you write $Match.
• On a $Match miss the suffix decides the fallback: ~ merges at the index, + appends (upsert), a bare key does nothing. Pair $Match with ~ or + when you want a defined fallback.
• SubCategories+ inside the matched element shows that operators compose to any depth. See the syntax reference for the full ruleset.
• A plain array with no $Match and no ~/+ still replaces wholesale, exactly as before.