de.ipcon.db.castor

Class PrefetchEvaluator

java.lang.Object

de.ipcon.db.castor.MEXTransformerEvaluator

de.ipcon.db.castor.PrefetchEvaluator

All Implemented Interfaces:

MEXTransformerEvaluatorI

public class PrefetchEvaluator
extends MEXTransformerEvaluator

MEXTransformerEvaluator class responsible for handling "Prefetch" instructions within a MEX template.

The Prefetch instruction allows for "Eager Loading" of object relationships (attribute chains). By specifying paths here, the underlying persistence handler can fetch related entities in a single database round-trip, avoiding the "N+1 selects" problem during result iteration.

Example usage: "{Prefetch Orders, Orders.Items, Contacts}"

Note: Similar to Union operations, this evaluator returns an empty string as its primary purpose is to modify the execution state of the MEXPersistenceQueryResults context, not the query text itself.

Constructor Summary

Constructors

Constructor and Description
PrefetchEvaluator(MEXPersistenceQueryResults _mexQR) Constructs a new PrefetchEvaluator.

Method Summary

Modifier and Type	Method and Description
boolean	canHandle(java.lang.CharSequence s) Determines if the expression is a Prefetch instruction.
java.lang.CharSequence	evaluate(java.lang.CharSequence s, MEXTransformer transformer) Parses and evaluates the Prefetch instruction.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

PrefetchEvaluator

public PrefetchEvaluator(MEXPersistenceQueryResults _mexQR)

Constructs a new PrefetchEvaluator.

Parameters:

_mexQR - the MEX query results context where prefetch paths will be registered.

Method Detail

canHandle

public boolean canHandle(java.lang.CharSequence s)

Determines if the expression is a Prefetch instruction.

Specified by:

canHandle in class MEXTransformerEvaluator

Parameters:

s - the expression string to check.

Returns:

true if it begins with "Prefetch".

evaluate

public java.lang.CharSequence evaluate(java.lang.CharSequence s,
                                       MEXTransformer transformer)

Parses and evaluates the Prefetch instruction.

The method strips the "Prefetch" tag, splits the remaining string by commas, validates each path, and registers it with the parent query context.

Specified by:

evaluate in interface MEXTransformerEvaluatorI

Specified by:

evaluate in class MEXTransformerEvaluator

Parameters:

s - the instruction sequence (e.g. "Prefetch a, b.c").

transformer - the active transformer.

Returns:

an empty string to remove the instruction from the final query text.

Throws:

MEXUnavailableException - if a provided path is syntactically invalid (e.g. contains illegal characters or empty segments).