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HTML
419 lines
20 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Creating Indexes</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Getting Started with Berkeley DB Java Edition" />
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<link rel="up" href="persist_index.html" title="Chapter 4. Working with Indices" />
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<link rel="prev" href="persist_index.html" title="Chapter 4. Working with Indices" />
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<link rel="next" href="persist_access.html" title="Chapter 5. Saving and Retrieving Objects" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 12.2.7.5</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Creating Indexes</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="persist_index.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 4. Working with Indices</th>
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<td width="20%" align="right"> <a accesskey="n" href="persist_access.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="dplindexcreate"></a>Creating Indexes</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<dl>
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<dt>
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<span class="sect2">
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<a href="dplindexcreate.html#dplprimaryidxdecl">Declaring Primary Indexes</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="dplindexcreate.html#dplsecondaryidxdecl">Declaring Secondary Indexes</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="dplindexcreate.html#foreignkey">Foreign Key Constraints</a>
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</span>
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</dt>
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</dl>
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</div>
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<p>
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To create an index using the DPL, you use Java
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annotations to declare which feature on the class is used
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for the primary index, and which features (if any) are to
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be used as secondary indexes.
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</p>
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<p>
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All entity classes stored in the DPL must have a
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primary index declared for it.
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</p>
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<p>
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Entity classes can have zero or more secondary
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indexes declared for them. There is no limit on the
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number of secondary indexes that you can declare.
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</p>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="dplprimaryidxdecl"></a>Declaring Primary Indexes</h3>
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</div>
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</div>
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</div>
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<p>
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You declare a primary key for an entity class by
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using the <code class="literal">@PrimaryKey</code>
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annotation. This annotation must appear
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immediately before the data member which
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represents the class's primary key. For example:
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</p>
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<pre class="programlisting">package persist.gettingStarted;
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import com.sleepycat.persist.model.Entity;
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import com.sleepycat.persist.model.PrimaryKey;
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@Entity
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public class Vendor {
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private String address;
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private String bizPhoneNumber;
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private String city;
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private String repName;
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private String repPhoneNumber;
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private String state;
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// Primary key is the vendor's name
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// This assumes that the vendor's name is
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// unique in the database.
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@PrimaryKey
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private String vendor;
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... </pre>
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<p>
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For this class, the <code class="literal">vendor</code> value is set for an individual
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<code class="classname">Vendor</code> class object by
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the <code class="methodname">setVendorName()</code>
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method. If our example code fails to set this
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value before storing the object, the data
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member used to store the primary key is set to a
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null value. This would result in a runtime
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error.
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</p>
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<p>
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You can avoid the need to explicitly set a
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value for a class's primary index by specifying
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a sequence to be used for the primary key. This
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results in an unique integer value being used
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as the primary key for each stored object.
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</p>
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<p>
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You declare a sequence is to be used by specifying
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the <code class="literal">sequence</code> keyword to the
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<code class="literal">@PrimaryKey</code> annotation. You must
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also provide a name for the sequence. For example:
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For example:
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</p>
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<pre class="programlisting">@PrimaryKey(sequence="Sequence_Namespace")
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long myPrimaryKey; </pre>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="dplsecondaryidxdecl"></a>Declaring Secondary Indexes</h3>
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</div>
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</div>
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</div>
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<p>
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To declare a secondary index, we use the
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<code class="literal">@SecondaryKey</code> annotation. Note
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that when we do this, we must declare what sort of
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an index it is; that is, what is its relationship to
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other data in the data store.
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</p>
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<p>
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The <span class="emphasis"><em>kind</em></span> of indices that we
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can declare are:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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<code class="literal">ONE_TO_ONE</code>
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</p>
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<p>
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This relationship indicates that
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the secondary key is unique to the
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object. If an object is stored with a
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secondary key that already
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exists in the data store, a run
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time error is raised.
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</p>
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<p>
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For example, a person object might
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be stored with a primary key of a
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social security number (in the US),
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with a secondary key of the
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person's employee number. Both
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values are expected to be unique in
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the data store.
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</p>
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</li>
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<li>
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<p>
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<code class="literal">MANY_TO_ONE</code>
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</p>
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<p>
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Indicates that the secondary key
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may be used for multiple
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objects in the data store. That is,
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the key appears more than
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once, but for each stored object it
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can be used only once.
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</p>
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<p>
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Consider a data store that relates
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managers to employees. A given
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manager will have multiple
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employees, but each employee is
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assumed to have just one manager.
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In this case, the manager's
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employee number might be a
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secondary key, so that you can
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quickly locate all the objects
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related to that manager's
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employees.
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</p>
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</li>
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<li>
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<p>
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<code class="literal">ONE_TO_MANY</code>
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</p>
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<p>
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Indicates that the secondary key
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might be used more than once for a
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given object. Index keys
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themselves are assumed to be
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unique, but multiple instances of
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the index can be used per object.
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</p>
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<p>
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For example, employees might have
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multiple unique email addresses. In
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this case, any given object can be
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access by one or more email
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addresses. Each such address is
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unique in the data store, but each
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such address will relate to a
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single employee object.
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</p>
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</li>
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<li>
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<p>
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<code class="literal">MANY_TO_MANY</code>
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</p>
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<p>
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There can be multiple keys for
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any given object, and for any given
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key there can be many related
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objects.
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</p>
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<p>
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For example, suppose your
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organization has a shared
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resource, such as printers. You
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might want to track which
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printers a given employee can
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use (there might be more than
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one). You might also want to
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track which employees can use a
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specific printer. This
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represents a many-to-many
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relationship.
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</p>
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</li>
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</ul>
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</div>
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<p>
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Note that for <code class="literal">ONE_TO_ONE</code> and
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<code class="literal">MANY_TO_ONE</code> relationships, you
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need a simple data member (not an array or
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collection) to hold the key. For
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<code class="literal">ONE_TO_MANY</code> and
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<code class="literal">MANY_TO_MANY</code> relationships, you
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need an array or collection to hold the keys:
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</p>
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<pre class="programlisting">@SecondaryKey(relate=ONE_TO_ONE)
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private String primaryEmailAddress = new String();
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@SecondaryKey(relate=ONE_TO_MANY)
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private Set<String> emailAddresses = new HashSet<String>(); </pre>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="foreignkey"></a>Foreign Key Constraints</h3>
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</div>
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</div>
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</div>
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<p>
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Sometimes a secondary index is related in some
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way to another entity class that is also
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contained in the data store. That is, the
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secondary key might be the primary key for
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another entity class. If this is the case, you
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can declare the foreign key constraint to make
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data integrity easier to accomplish.
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</p>
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<p>
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For example, you might have one class that is
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used to represent employees.
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You might have another that is used to
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represent corporate divisions. When you add or
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modify an employee record, you might want to
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ensure that the division to which the employee
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belongs is known to the data store. You do this
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by specifying a foreign key constraint.
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</p>
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<p>
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When a foreign key constraint is declared:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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When a new secondary key
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for the object is stored,
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it is checked to make sure
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it exists as a primary
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key for the related
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entity object. If it does
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not, a runtime error
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occurs.
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</p>
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</li>
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<li>
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<p>
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When a related entity is
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deleted (that is, a
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corporate division is
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removed from the data
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store), some action is
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automatically taken for
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the entities that refer to
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this object (that is, the
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employee objects). Exactly
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what that action is, is
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definable by you. See
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below.
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</p>
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</li>
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</ul>
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</div>
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<p>
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When a related entity is deleted from the data
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store, one of the following actions are taken:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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<code class="literal">ABORT</code>
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</p>
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<p>
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The delete operation is not
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allowed. A runtime error is
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raised as a result of the
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operation. This is the
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default behavior.
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</p>
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</li>
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<li>
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<p>
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<code class="literal">CASCADE</code>
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</p>
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<p>
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All entities related to this
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one are deleted as well. For
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example, if you deleted a
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<code class="classname">Division</code>
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object, then all
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<code class="classname">Employee</code>
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objects that belonged to the
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division are also deleted.
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</p>
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</li>
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<li>
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<p>
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<code class="literal">NULLIFY</code>
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</p>
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<p>
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All entities related to the
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deleted entity are updated so
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that the pertinent data member
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is nullified. That is, if you
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deleted a division, then all
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employee objects related to
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that division would have their
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division key
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automatically set to null.
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</p>
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</li>
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</ul>
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</div>
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<p>
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You declare a foreign key constraint by using
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the <code class="literal">relatedEntity</code> keyword. You
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declare the foreign key constraint deletion policy using the
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<code class="literal">onRelatedEntityDelete</code> keyword. For
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example, the following declares a foreign key
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constraint to <code class="classname">Division</code>
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class objects, and it causes related objects to
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be deleted if the <code class="classname">Division</code>
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class is deleted:
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</p>
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<pre class="programlisting">@SecondaryKey(relate=ONE_TO_ONE, relatedEntity=Division.class,
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onRelatedEntityDelete=CASCADE)
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private String division = new String(); </pre>
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</div>
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||
</div>
|
||
<div class="navfooter">
|
||
<hr />
|
||
<table width="100%" summary="Navigation footer">
|
||
<tr>
|
||
<td width="40%" align="left"><a accesskey="p" href="persist_index.html">Prev</a> </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="u" href="persist_index.html">Up</a>
|
||
</td>
|
||
<td width="40%" align="right"> <a accesskey="n" href="persist_access.html">Next</a></td>
|
||
</tr>
|
||
<tr>
|
||
<td width="40%" align="left" valign="top">Chapter 4. Working with Indices </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="h" href="index.html">Home</a>
|
||
</td>
|
||
<td width="40%" align="right" valign="top"> Chapter 5. Saving and Retrieving Objects</td>
|
||
</tr>
|
||
</table>
|
||
</div>
|
||
</body>
|
||
</html>
|