constraint: constraint
In cboettig/build.eml: Build Ecological Metadata Language Files

constraint

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constraint(primaryKey = NULL, uniqueKey = NULL,
  checkConstraint = NULL, foreignKey = NULL, joinCondition = NULL,
  notNullConstraint = NULL, id = NULL, system = NULL, scope = NULL)

`primaryKey`	The primary key in the entity See `primaryKey()`
`uniqueKey`	A unique key in the entity See `uniqueKey()`
`checkConstraint`	A constraint which checks a conditional clause within an entity. See `checkConstraint()`
`foreignKey`	A foreign key relationship among entities See `foreignKey()`
`joinCondition`	A non primary/foreign key join See `joinCondition()`
`notNullConstraint`	A constraint that indicates that no null values should be present for an attribute. See `notNullConstraint()`
`id`	A unique identifier for this additional metadata that can be used to reference it elsewhere. This is a formal field in that it is an error to provide a value for the id attribute that is not unique within the document's set of id attributes. This is designed to allow other portions of the metadata to reference this section formally.
`system`	The data management system within which an identifier is in scope and therefore unique. This is typically a URL (Uniform Resource Locator) that indicates a data management system. All identifiers that share a system must be unique. In other words, if the same identifier is used in two locations with identical systems, then by definition the objects at which they point are in fact the same object.
`scope`	The scope of the identifier. Scope is generally set to either "system", meaning that it is scoped according to the "system" attribute, or "document" if it is only to be in scope within this single document instance. In this particular use of scope, it is FIXED to be "system" because the packageId is required and always has the scope of the required "system".