Figure 1210 A model that causes referential integrity problems for Concrete Table Inheritance in Java

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Figure 1210 A model that causes referential integrity problems for Concrete Table Inheritance
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If you're searching for players with a select statement, you need to look at all tables to see which ones contain the appropriate value This means using multiple queries or using an outer join, both of which are bad for performance You don't suffer the performance hit when you know the class you need, but you do have to use the concrete class to improve performance
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This pattern is often referred to as along the lines of leaf table inheritance Some people prefer a variation where you have one table per leaf class instead of one table per concrete class If you don't have any concrete superclasses in the hierarchy, this ends up as the same thing Even if you do have concrete superclasses the
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difference is pretty minor
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When figuring out how to map inheritance, Concrete Table Inheritance, Class Table Inheritance (285), and Single Table Inheritance (278) are the alternatives
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The strengths of Concrete Table Inheritance are:
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Each table is self-contained and has no irrelevant fields As a result it makes good sense when used by other applications that aren't using the objects There are no joins to do when reading the data from the concrete mappers Each table is accessed only when that class is accessed, which can spread the access load
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The weaknesses of Concrete Table Inheritance are:
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Primary keys can be difficult to handle You can't enforce database relationships to abstract classes If the fields on the domain classes are pushed up or down the hierarchy, you have to alter the table definitions You don't have to do as much alteration as with Class Table Inheritance (285), but you can't ignore this as you can with Single Table Inheritance (278) If a superclass field changes, you need to change each table that has this field because the superclass fields are duplicated across the tables A find on the superclass forces you to check all the tables, which leads to multiple database accesses (or a weird join)
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Remember that the trio of inheritance patterns can coexist in a single hierarchy So you might use Concrete Table Inheritance for one or two subclasses and Single Table Inheritance (278) for the rest
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Example: Concrete Players (C#)
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Here I'll show you an implementation for the sketch As with all inheritance examples in this chapter, I'm using the basic design of classes from Inheritance Mappers (302), shown in Figure 1211
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Figure 1211 The generic class diagram of Inheritance Mappers (302)
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Each mapper is linked to the database table that's the source of the data In ADONET a data set holds the data table
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class Mapper public Gateway Gateway; private IDictionary identityMap = new Hashtable(); public Mapper (Gateway gateway) { thisGateway = gateway; } private DataTable table { get {return GatewayDataTables[TableName];} } abstract public String TableName {get;}
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The gateway class holds the data set within its data property The data can be loaded up by supplying suitable queries
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class Gateway public DataSet Data = new DataSet();
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Each concrete mapper needs to define the name of the table that holds its data
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class CricketerMapper public override String TableName { get {return "Cricketers";} }
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The player mapper has fields for each concrete mapper
class PlayerMapper private BowlerMapper bmapper; private CricketerMapper cmapper; private FootballerMapper fmapper; public PlayerMapper (Gateway gateway) : base (gateway) { bmapper = new BowlerMapper(Gateway); cmapper = new CricketerMapper(Gateway); fmapper = new FootballerMapper(Gateway); }
Loading an Object from the Database
Each concrete mapper class has a find method that returns an object given a key value
class CricketerMapper public Cricketer Find(long id) { return (Cricketer) AbstractFind(id); }
The abstract behavior on the superclass finds the right database row for the ID, creates a new domain object of the correct type, and uses the load method to load it up (I'll describe the load in a moment)
class Mapper public DomainObject AbstractFind(long id) { DataRow row = FindRow(id); if (row == null) return null; else { DomainObject result = CreateDomainObject(); Load(result, row); return result; } } private DataRow FindRow(long id) { String filter = StringFormat("id = {0}", id); DataRow[] results = tableSelect(filter); if (resultsLength == 0) return null; else return results[0]; } protected abstract DomainObject CreateDomainObject(); class CricketerMapper protected override DomainObject CreateDomainObject(){ return new Cricketer(); }
The actual loading of data from the database is done by the load method, or rather by several load methods: one each for the mapper class and for all its superclasses
class CricketerMapper protected override void Load(DomainObject obj, DataRow row) { baseLoad(obj,row); Cricketer cricketer = (Cricketer) obj; cricketerbattingAverage = (double)row["battingAverage"]; } class AbstractPlayerMapper
protected override void Load(DomainObject obj, DataRow row) { baseLoad(obj, row); Player player = (Player) obj; playername = (String)row["name"]; class Mapper protected virtual void Load(DomainObject obj, DataRow row) { objId = (int) row ["id"]; }
This is the logic for finding an object using a mapper for a concrete class You can also use a mapper for the superclass: the player mapper, which it needs to find an object from whatever table it's living in Since all the data is already in memory in the data set, I can do this like so:
class PlayerMapper public Player Find (long key) { Player result; result = fmapperFind(key); if (result != null) return result; result = bmapperFind(key); if (result != null) return result; result = cmapperFind(key); if (result != null) return result; return null; }
Remember, this is reasonable only because the data is already in memory If you need to go to the database three times (or more for more subclasses) this will be slow It may help to do a join across all the concrete tables, which will allow you to access the data in one database call However, large joins are often slow in their own right, so you'll need to do some benchmarks with your own application to find out what works and what doesn't Also, this will be an outer join, and as well as slowing the syntax it's nonportable and often cryptic Updating an Object