67 Concepts

    68 ~~~~~~~~

    69 

    70 Entity type

    71 ```````````

    72 

    73 RQL manipulates variables that are instances of entities.

    74 Each entity has its own type which are used in backend to improve the query

    75 execution plan.

    76 

    77 Restrictions

    78 ````````````

    79 

    80 They are conditions used to limit the perimeter of the result set.

    81 

    82 Relations

    83 `````````

    84 A relation is a `3-expression` defined as follows:

    85 

    86 .. image:: Graph-ex.gif

    87     :alt: <subject> <predicate> <object>

    88     :align: center

    89 

    90 A RQL relation contains three components:

    91 

    92 * the subject, which is an entity type

    93 * the predicate, which is an oriented graph

    94 * the object, which is either an attribute or a relation to another entity

    95 

    96 In cubicweb, the term `relation` is often found without ambiguity instead of `predicate`.

    97 This predicate is also known as the `property` of the triple in `RDF concepts`_

    98 

    99 A relation is always expressed in the order: subject, predicate, object.

   100 

   101 It's important to determine if entity type is subject or object to construct a

   102 valid expression. An inversion subject/object is equivalent to an RQL error

   103 since the supposed relation cannot be found in schema. If you don't have access

   104 to the code, you could find the order by looking at the schema image in manager

   105 views (the subject is located at the beginning of the arrow).

   106 

   107 .. _SQL: http://www.firstsql.com/tutor5.htm

   108 .. _RDF concepts: http://www.w3.org/TR/rdf-concepts/

   109 

   110 Cardinality

   111 ```````````

   112 XXX

   113 

   114 Cardinality is an important concept to model your business rules.

   115 They determine nu./tutorials/advanced/index.rst

   116 

   117 Please refer to the `datamodel definitions`_ for a deep understanding.

   118 

   119 `Relations`_ are always expressed by cardinality rules (`**` by default)

   120 

   121 .. _datamodel definitions: ./devrepo/datamodel/definition.rst

   122 

   123 Transaction

   124 ```````````

   125 

   126 RQL supports notion of **transactions**; i.e. sequences of RQL statements

   127 without invoking security hooks of the instance's schema.

   128 

   129 When you're ready to make persistent the changes, you have to *commit* the

   130 modification in calling `commit()`.

   131 

   132 If an error is found (typically in raising a ValidationError), you have the

   133 possibility to roll back the transaction in invoking `rollback()` function; i.e

   134 to come back to the initial state of the transaction.

   135 

   136 Please, refer to the :ref:`Migration` chapter if you want more details.