[ROOT] / doc / toc / ARCQuery / Class / CompoundKey

FunctionKeys , Parse , QuantileKey , ToString , TryGetP , TryGetPInternal

A class able to expand the concept of entity keys.

Example: In addition to ordinary keys for 'Customer' like 'FirstName', 'LastName' and 'DateOfBirth', it is able to offer:
1) Expressions like "FirstName'+' '+'LastName" (see BinaryOperatorKey and ConstantKey<T>).
2) Keys implemented directly in actual class like 'Name' (see EntityMethodKey).
3) Functions acting on keys, like 'DateOfBirth.Year()' (see FunctionKey / FunctionKeyYear).
4) Global new keys like 'CountP()' (see NewKey / NewKeyCountP).
5) Foreign keys with for instance aggregations like 'Order.Amount.Sum()' (see ForeignKey / FunctionKeyAggregate / -FunctionKeyAggregateSum).

These keys / expressions can then be used whenever ordinary keys are used, like in QueryExpressions like SELECT, WHERE, AGGREGATE, PIVOT and so on.

Most relevant sub classes of CompoundKey are:

1) BinaryOperatorKey: Offers expressions with operators like +, -, * and /.

2) ConstantKey<T>: Represents a constant expression like "' '", "', '", "42".

3) EntityMethodKey: Key able to call read-only methods defined in the entity class with names like 'TryGet{FieldName}'.
Example: For 'Customer' it can find 'Customer.Name' which is calculated in a read-only method Customer.TryGetName as combination of 'FirstName', 'LastName' and 'CompanyName'.

4) NewKey: Creates new keys available for all entities.
Example: For 'Customer' it can count number of properties through 'CountP' (see NewKeyCountP.)

5) ForeignKey: Can find related values.
Example: For 'Order', it can find 'Customer.Name'.

6) MemberEntityKey: Can find value for entity which is member of entity being queried.
Example: For 'Order', it can find 'Customer.Name'.

Closely related classes (Stored inside CompoundKey as members FunctionKeys, and QuantileKey) are:
1) FunctionKey / FunctionKeyAggregate: Extracts data from an already existing value like 'Created.Year()' (see FunctionKeyYear)
and
2) QuantileKey: Can separate entities into quantiles, like Quartiles, Quintiles or Sextiles

(See also ValueComparer which can evaluation expressions like for 'Customer': 'WHERE Created = ThisYear')

Other sub-classes of CompoundKey are OrdinaryKeyPlusFunctionOrQuantile and CompoundInvalidKey.

See common parser for all compound keys (Parse) which will always 'succeed' (a 'TryParse'-method is neither needed nor offered, CompoundInvalidKey is returned if not key found).

Note how compound keys (and related keys) may be chained several levels deep, like for 'Order': 'Customer.Created.Year()' or for 'Customer': 'Order.Amount.Sum()' or even 'Order.Amount.Sum().QUARTILE'.
This is all parsed in Parse and stored in FunctionKeys / QuantileKey.

See QueryExpressionSelect for hint about using 'AS'-clause in order to simplify parsing and evaluation in subsequent QueryExpressions when using CompoundKey.

All keys can be followed by one or several FunctionKeys

A QuantileKey can only be the last key in a chain, like 'Customer.Order.Amount.Sum().QUARTILE'.
It is not evaluated by CompoundKey.TryGetP, it is instead evaluated by ARCQuery. Extensions.TryGetP)

Generated 2024-04-18 23:49:36.210 UTC