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Template:SAIF Generic IM Constraints

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Generic Content Constraints for SAIF Information models.
         Only the content appearing below in black font will appear in the inclusion.
These elements are intended to appear under the following section:

Content Constraints

For Information Models that are derived from either the HL7 Reference Information Model (RIM) or from an information model that has the RIM of as one of its derivation ancestors, the Content Constraints are simply stated:

The contents of this model must be either the same as the contents of the model from which it is derived from or a constrained sub-set of the contents of that model.

The key question is what constitutes a "constrained sub-set" of the contents of a model? The answer to these rules is found in HL7 Version 3 Standard: Refinement, Constraint and Localization to Version 3 Messages, Release 2 (ANSI/HL7 V3 RCL, R2-2007), approved in August 2007. (Do not be mis-led by the term "Messages" in the title of this standard. A "message" is simply a Serializable Information Model (SIM).)

The remainder of this section will summarize the constraint rules that are documented in that standard, and modify them to use SAIF terms. It does not replace those rules. The process of constraining, in general, reduces the "breadth" of the semantic content of the model, or limits the scope of the values for a particular element.

Constraints from Refinement, Constraint and Localization Standard

The key elements of the governing standard are in chapter 2 - Constraints and Annotations which begins as follows. Constraints are the central feature of the derivation process, as they reduce the generality of the base model and focus it on a particular requirement. The constraints that may be asserted against a model element, e.g. class, attribute or association, fall into six broad categories:

Appearance constraints determine whether a particular model element in the base model must appear in models or messages derived from the base model, and/or whether the element is precluded from appearing therein.

Cardinality constraints define the number of repetitions that may occur for a given element.

Type constraints limit the structure or "type" of the element in question. These are constraints placed upon the data types for attributes and upon the use of Common Model Element Types (CMETs) at association ends.

Vocabulary constraints limit the set of concepts that can be taken as valid values in an instance of a coded attribute or data type.

Other value constraints provide for the declaration of constraints stated as text and, optionally, as testable expressions to establish "business rules", and for the assertion of default or fixed values.

Appearance Constraints

Appearance constraints apply to attributes and associations. The presence of a model element in a derived model depends upon the appearance constraints that are asserted or implied for that element in the base model, and upon the rules for "cloning" classes from a source model to a target model.

There are two forms of appearance constraints that may be set in a model: the declaration that an element is mandatory; and the assertion of a conformance value.

Mandatory Inclusion Property

The mandatory inclusion property indicates whether or not a particular element must be present in each instance of serialized model. If the mandatory inclusion property of a particular element is set to "M", the element is mandatory. If an element is Mandatory, then all elements that derive from it in derived models SHALL also be Mandatory.

Conformance Property

The conformance property specifies how a model element must be handled by applications. Every attribute and association element in a model has an explicit or implicit conformance property. The property has one of three values: Required, Unspecified or Not Permitted:

  • Required (R) - When deriving models, the element must appear in all instances of derived models and must be declared as "Required" in derived models.
  • Unspecified (U) - The appearance of this element in derived models is unconstrained. In instances of derived models the element may retain "Unspecified" status, or it may be declared "Required" or "Not Permitted". "Unspecified" is the default value for this property.
  • Not Permitted (NP) - This element SHALL not appear in derived models or, it it appears, it MUST also be "Not permitted" in the derived model.

Cloning classes

Whenever an information model is derived by constraining another information model, it is permissible to "clone" the classes of the base model. This is true whether one is deriving a DIM from the RIM, a SIM from a DIM, or an SIM from another SMIM.

Class cloning is the creation of one or more copies of a base class contained in the source model into the new model. Cloning is the only form of model "extension" permitted under the terms of this section. The following rules apply:

  • In order to be a source class for more than one clone in the derived model, a class in the base model:
    • Must be reached by an association whose upper cardinality limit is greater than "1", or that is reached by a "recursive" relationship ( typically when a class (type) is declared to have a "component" relationship to its self).
  • In order to qualify as a valid clone of a source class, the clone must obey the following rules:
    • If the source class has been cloned more than one in the derivation, the clone classes SHALL have a new name to assure that they have unique names within the derived model.
    • The clone may contain only attributes that are also part of the source class
    • The clone shall only use associations that are valid for the source class and are present in the base model
    • The cardinality and mandatory constraints for elements in the clone class cannot be less constrained than the equivalent elements in the source class
    • The vocabulary domains declared for any coded attributes in the clone must be identical to, or a subset of, the domain asserted in the source class, and if the coded attribute is "CNE" the cloned attribute must also be "CNE"
    • The clone need not include attributes or associations unless they are "Required" or "Mandatory" in the source model, regardless of their cardinality
    • The clone may not include attributes or associations that have a conformance value of "Not Permitted" in the base model

Cardinality constraints

Cardinality constraints apply only to attributes and associations. Every attribute or association element in a model has an explicit or implicit cardinality that expresses the number of times an attribute or association may appear in a derived model or model instance. Since associations in HL7 information models are directional, the cardinality constraint of an association is equal to its destination multiplicity, i.e. the minimum and maximum number of times instances of the target class of the association may be referenced.

When deriving a model both the minimum and maximum cardinality for a particular derived element may be constrained in a way that narrows the range of possible occurrences. Thus, either minimum or maximum values of the derived cardinality range may be set to a new value within the closed range defined by the minimum and maximum values in the source range. This is subject to two the further constraint rule that the minimum cardinality of the derived range must be less than or equal to the maximum cardinality of that range.