# MgGrid — Master-Detail

> Part of the MgGrid system. See `README.md` for overview and component hierarchy.

## How It Works

1. `MgGridBase.BuildRenderTree` wraps content in `CascadingValue<IMgGridBase>`
2. Child grids receive this via `[CascadingParameter] IMgGridBase? ParentGrid`
3. `IsMasterGrid` = `ParentDataItem == null`
4. `GetRootGrid()` walks the `ParentGrid` chain to find the topmost grid

## Detail Grid Setup

```razor
<DetailRowTemplate>
    @{
        var parent = (ParentEntity)context.DataItem;
        <GridChildEntity ParentDataItem="@parent"
                         KeyFieldNameToParentId="ParentEntityId"
                         ContextIds="@(new object[] { parent.Id })" />
    }
</DetailRowTemplate>
```

When `ParentDataItem` is set and `KeyFieldNameToParentId` is provided, new items automatically get their parent FK set via reflection.