Tables for
Volume G
Definition and exchange of crystallographic data
Edited by S. R. Hall and B. McMahon

International Tables for Crystallography (2006). Vol. G, ch. 3.1, p. 82

Section Inheritance of identifiers

B. McMahona*

aInternational Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England
Correspondence e-mail: Inheritance of identifiers

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Example[link] is from an mmCIF of two related categories that describe characteristics of an active site in a macromolecular complex. The sites are described in general terms with a label and textual description in the STRUCT_SITE category (the first looped list in the example). Details of how each site is generated from a list of structural features form the STRUCT_SITE_GEN category (second loop or table).

Example Illustration of parent/child relationships between identifiers in related categories.

[Scheme scheme15]

It is clear that each instance of the data item _struct_site_gen.site_id in the second table must have one of the values listed as in the first loop, because it is the purpose of these identifiers to relate the two sets of data: they are the glue between the two separate tables and must have the same values to ensure the referential integrity of the data set (that is, the consistency and completeness of cross-references between tables). Within a group of related categories like this, it is normal to consider one as the `parent' and the others as `children'.

Because all such linking data items must have compatible attributes, it is conventional in DDL2 dictionaries to define all the attributes in a single location, namely the save frame which hosts the definition of the `parent' data item. In early drafts of DDL2 dictionaries, the `children' were not referenced at all in separate save frames; software validating a data file against a dictionary was required to obtain all information about a child identifier from the contents of the save frame defining the parent. However, subsequent drafts introduced a minimal save frame for the children to accommodate dictionary browsers that depended on the existence of a separate definition block for each individual data item.

Consequently, the definition blocks in current DDL2 dictionaries conform to the structure in Example[link], which refers to the simple STRUCT_SITE example used above.

Example A definition of an identifier which is parent to identifiers in other categories.

[Scheme scheme16]

Note that the dependent data names are listed twice: once in the loop that declares their values and the categories with which they are associated; and again in a loop that makes the direction of the relationship explicit. A parent data item may have several children, but each child can have only a single parent (i.e. related data name whose value may be checked for referential integrity). Note also that each listed item has an _item.mandatory_code value of yes: because they are identifiers which link categories, they must be present in a table to allow the relationships between data items in different tables to be traced.

Other than the specific description text field, any declared attributes (in this example only the data type) have a common value across the set of related identifiers.

As mentioned above, it is not formally necessary to have a separate save frame for the individual children; but it is conventional to have such individual save frames containing minimal definitions that serve as back references to the primary information in the parent frame. These also provide somewhere for the specific text definitions for the children to be stored. The definition frame for is shown in Example[link].

Example Definition of a child identifier.

[Scheme scheme17]

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