Wolfram Language & System 12.1 (2020)|Legacy Documentation
EntityList
Listing of Entity Types »EntityList[class]
gives a list of entities in the specified entity class.
EntityList["type"]
gives a list of entities of the specified type.
EntityList[class,simplify]
gives a list of entities; simplify determines whether to reduce entities to the simplest possible type.
Details
- In EntityList[class], class can be one of the following:
-
EntityClass representation of a table or virtual table in a database FilteredEntityClass entity class filtered by a criterion SortedEntityClass entity class sorted by properties or functions SampledEntityClass entity class formed by sampling a number of entities ExtendedEntityClass entity class with properties added for each entity AggregatedEntityClass entity class formed by aggregating entities CombinedEntityClass entity class formed by combining properties across entity classes - EntityList["type"] is systematically supported only for types allowing a fixed number of entities.
- EntityList[class] is effectively equivalent to EntityList[class,True].
- When reducing entities to their base type, the outermost layers of EntityClass, FilteredEntityClass, SortedEntityClass and SampledEntityClass can be removed; anything else must be kept.
Examples
open allclose allBasic Examples (4)
EntityList can also be used on EntityClass expressions:
EntityList can be used on many types of entity classes:
Use EntityList with a second argument to decide whether to simplify the entities:
Scope (2)
Properties & Relations (6)
EntityList can be used with FilteredEntityClass:
EntityList can be used with SampledEntityClass:
EntityList can be used with SortedEntityClass:
EntityList can be used with ExtendedEntityClass, but it will not simplify:
When other heads that can be simplified appear around ExtendedEntityClass, they will be simplified:
EntityList can be used with AggregatedEntityClass, but it will not simplify:
EntityList can be used with CombinedEntityClass, but it will not simplify:
Possible Issues (3)
In general, it is best to avoid calling EntityList before EntityValue, as two separate calls will have to be performed:
This will have consequences in performance, and also in atomicity when dealing with external databases:
When EntityList returns a list of Entity objects with complex first arguments, those might stop existing if the data changes:
Update the value of "p1" so that the condition is no longer met:
When dealing with external databases, one might encounter a database where the primary key was not set for some tables:
When this happens, EntityStore will emit a message. This means that some functionality involving single entities will be disabled:
But EntityValue will generally keep working:
The only way around this problem is to set the primary key constraint in the external database.