.. index:: Core Operations; Occurrence Attachment
.. _core-operation-occurrence-attachment:

OccurrenceAttachment
==================================

Applies an attachment to the sum of a group of records that share equal values
for their ``Trial``, ``Time``, and ``OccurrenceKey`` fields. Once the attachment is
applied, the result is proportionally allocated to all records in the group.

Structure
---------

.. code-block:: json

    {
        "_schema": "OccurrenceAttachment_1.0",
        "attachment": 1000,
        "currency": "GBP"
    }

Parameters
----------

+----------------------+----------+------------------+-------------------------------------------------------------+
| Parameter Name       | Required | Type             | Description                                                 |
+======================+==========+==================+=============================================================+
| ``attachment``       | Yes      | ``double``       | The attachment value in currency units. Cannot be negative. |
+----------------------+----------+------------------+-------------------------------------------------------------+
| ``currency``         | No       | ``string``       | The currency in which ``attachment`` is defined.            |
+----------------------+----------+------------------+-------------------------------------------------------------+

Behaviour
--------------

This is equivalent to the :doc:`scoped_attachment` with
``span_columns`` set to ``[Time, OccurrenceKey]`` and ``erode_columns`` set to
``[Time, OccurrenceKey]``.
In other words, it applies a non-eroding attachment to occurrences generated by
combining records with matching ``Time`` and ``OccurrenceKey`` values.