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The canonical case for Association Table Mapping is a many-to-many association, since there are really no any alternatives for that situation
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Association Table Mapping can also be used for any other form of association However, because it's more complex than Foreign Key Mapping (236) and involves an extra join, it's not usually the right choice Even so, in a couple of cases Association Table Mapping is appropriate for a simpler association; both involve databases where you have less control over the schema Sometimes you may need to link two existing tables, but you aren't able to add columns to those tables In this case you can make a new table and use Association Table Mapping Other times an existing schema uses an associative table, even when it isn't really necessary In this case it's often easier to use Association Table Mapping than to simplify the database schema
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In a relational database design you may often have association tables that also carry information about the relationship An example is a person/company associative table that also contains information about a person's employment with the company In this case the person/company table really corresponds to a true domain object
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Example: Employees and Skills (C#)
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Here's a simple example using the sketch's model We have an employee class with a collection of skills, each of which can appear for more than one employee
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class Employee public IList Skills { get {return ArrayListReadOnly(skillsData);} set {skillsData = new ArrayList(value);} } public void AddSkill (Skill arg) { skillsDataAdd(arg);
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} public void RemoveSkill (Skill arg) { skillsDataRemove(arg); } private IList skillsData = new ArrayList();
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To load an employee from the database, we need to pull in the skills using an employee mapper Each employee mapper class has a find method that creates an employee object All mappers are subclasses of the abstract mapper class that pulls together common services for the mappers
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class EmployeeMapper public Employee Find(long id) { return (Employee) AbstractFind(id); } class AbstractMapper protected DomainObject AbstractFind(long id) { AssertTrue (id != DomainObjectPLACEHOLDER_ID); DataRow row = FindRow(id); return (row == null) null : Load(row); } protected DataRow FindRow(long id) { String filter = StringFormat("id = {0}", id); DataRow[] results = tableSelect(filter); return (resultsLength == 0) null : results[0]; } protected DataTable table { get {return dshDataTables[TableName];} } public DataSetHolder dsh; abstract protected String TableName {get;} class EmployeeMapper protected override String TableName { get {return "Employees";} }
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The data set holder is a simple object that contains an ADONET data set and the relevant adapters to save it to the database
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class DataSetHolder public DataSet Data = new DataSet(); private Hashtable DataAdapters = new Hashtable();
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To make this example simple-indeed, simplistic we'll assume that the data set has already been loaded with all the data we need
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The find method calls load methods to load data for the employee
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class AbstractMapper protected DomainObject Load (DataRow row) { long id = (int) row ["id"]; if (identityMap[id] != null) return (DomainObject) identityMap[id]; else { DomainObject result = CreateDomainObject(); resultId = id; identityMapAdd(resultId, result);
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doLoad(result,row); return result; } } abstract protected DomainObject CreateDomainObject(); private IDictionary identityMap = new Hashtable(); abstract protected void doLoad (DomainObject obj, DataRow row); class EmployeeMapper protected override void doLoad (DomainObject obj, DataRow row) { Employee emp = (Employee) obj; empName = (String) row["name"]; loadSkills(emp); }
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Loading the skills is sufficiently awkward to demand a separate method to do the work
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class EmployeeMapper private IList loadSkills (Employee emp) { DataRow[] rows = skillLinkRows(emp); IList result = new ArrayList(); foreach (DataRow row in rows) { long skillID = (int)row["skillID"]; empAddSkill(MapperRegistrySkillFind(skillID)); } return result; } private DataRow[] skillLinkRows(Employee emp) { String filter = StringFormat("employeeID = {0}", empId); return skillLinkTableSelect(filter); } private DataTable skillLinkTable { get {return dshDataTables["skillEmployees"];} }
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To handle changes in skills information we use an update method on the abstract mapper
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class AbstractMapper public virtual void Update (DomainObject arg) { Save (arg, FindRow(argId)); } abstract protected void Save (DomainObject arg, DataRow row);
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