# Nest Resource Templates

Use nested resource templates when every instance should expose the same hierarchy. Each `add_child` call returns a child builder. Add that child's properties or children, then call `close_child()` to return to its parent.

``` python
with namespace.build_resource_template(
    name="Experiment",
    type_names=["experiment"],
) as experiment:
    sample = experiment.add_child("Sample", ["sample"])
    sample.add_properties({
        "identity": [
            {"name": "sample_id", "type": "str", "default": ""},
        ],
    })

    vial = sample.add_child("Vial", ["vial"])
    vial.add_properties({
        "storage": [
            {"name": "position", "type": "str", "default": ""},
        ],
    })
    sample = vial.close_child()
    experiment = sample.close_child()
```

The example defines two nesting levels below `Experiment`: `Sample` and `Vial`. Close builders from deepest child back to root so each builder's parent is explicit. A `with` block also saves the root template when it exits.


# Distinguish template and runtime children

**Template children:** predictable structure defined once by a template.

**Runtime children:** experiment-specific structure added to an instance.

Use template children for required hierarchy such as experiment, sample, and vial. Use runtime children when count or structure is discovered during an experiment and should not change the template for every future instance.


# Copy an existing resource into a group

Use runtime children to group an existing resource without changing its source or its template. Pass either its [ResourceSchema](../../reference/ResourceSchema.md#recap.ResourceSchema) or UUID to `add_child`:

``` python
with namespace.build_resource(resource_id=group.id) as group_builder:
    copied_sample = group_builder.add_child(sample)
```

The operation copies the selected resource and its full descendant subtree beneath the group. The copied root has `copied_from_id == sample.id`; descendants do not individually record lineage. A copied root keeps its source name, and a group cannot receive another child with the same name. Rename the source before grouping when that name already exists under the group.