======================
Relations
======================

Objects in Mesh model can relate to each other. Relation between objects is
done using relation attributes. The relation attribute defines type of the
relation, target object type and other parameters.

.. image:: images/relations1.png
   :width: 800

There are two groups of relations between objects in Mesh model.

Ownership relation
-------------------

Each object is owned by some other object (owner). It is called ownership
relation.

Ownership relations can be:
  - one-to-one
  - one-to-many (collection): where value of *many* is in range defined by
    *minimum* and *maximum cardinality* on relation **attribute definition** level.

By convention ownership relation attribute names start with 'has\_...', e.g.:
*has_PowerPlant*. Removing an object will remove recursively all of its owned
objects as well, if there were any.

Link relation
------------------

Link relation is a relation where one object may point to another object, but
does not own it. There is also a versioned link relation, which is a link
relation where the target object can change over time. It consists of a list of
pairs:

  - Target object identifier.
  - Timestamp which indicates start of the period where the target object is
    active (linked to), the target object is active until the next target object
    in the list, if any, becomes active.

.. image:: images/versioned_link_relation.png
   :width: 800

There are four types of link relation attributes:
  - one-to-one
  - one-to-many (collection)
  - versioned one-to-one
  - versioned one-to-many (collection)

Similar to **ownership relation** the value of *many* is in range defined by
*minimum* and *maximum cardinality* on relation attribute definition` level.

.. image:: images/versioned_link_relation_collection.png
   :width: 800

By convention link relation attribute names start with 'to\_...', e.g.:
'to_EnergyMarket'. Removing an object that had link relations to other objects
will NOT remove the linked objects.

.. image:: images/relations2.png
   :width: 800