Subversion-Projekte lars-tiefland.prado

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<h1>Parameter Maps and Inline Parameters</h1>

<p>Most SQL statements are useful because we can pass them values at runtime.
Someone wants a database record with the ID 42, and we need to merge that ID
number into a select statement. A list of one or more parameters are passed at
runtime, and each placeholder is replaced in turn. This is simple but labor
intensive, since developers spend a lot of time counting symbols to make sure
everything is in sync.</p>

<div class="note"><b class="tip">Note:</b>
Preceding sections briefly touched on inline parameters, which automatically
map properties to named parameters. Many iBATIS developers prefer this
approach. But others prefer to stick to the standard, anonymous approach to
SQL parameters by using parameter maps. Sometimes people need to retain the
purity of the SQL statements; other times they need the detailed specification
offered by parameter maps due to database or provider-specific information
that needs to be used.
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<h1>Parameter Map</h1>
<p>A Parameter Map defines an ordered list of values that match up with the
placeholders of a parameterized query statement. While the attributes
specified by the map still need to be in the correct order, each parameter is
named. You can populate the underlying class in any order, and the Parameter
Map ensures each value is passed in the correct order.</p>

<p>Parameter Maps can be provided as an external element and \emph{inline}.
The following example shows an external Parameter Map.</p>

<com:TTextHighlighter Language="xml" CssClass="source">
<parameterMap id="parameterMapIdentifier"
  [extends="[sqlMapNamespace.]parameterMapId"]>
  <parameter
    property ="propertyName"
    [column="columnName"]
    [dbType="databaseType"]
    [type="propertyCLRType"]
    [nullValue="nullValueReplacement"]
    [size="columnSize"]
    [precision="columnPrecision"]
    [scale="columnScale"]
    [typeHandler="class.name"]
  <parameter ... ... />
  <parameter ... ... />
</parameterMap>
</com:TTextHighlighter>

<p>In the above example, the parts in <tt>[brackets]</tt> are optional. The
<tt>parameterMap</tt> element only requires the id attribute.
The following example shows a typical <tt>&lt;parameterMap&gt;</tt>.</p>

<com:TTextHighlighter Language="xml" CssClass="source">
<parameterMap id="insert-product-param" class="Product">
  <parameter property="description" />
  <parameter property="id"/>
</parameterMap>

<statement id="insertProduct" parameterMap="insert-product-param">
  insert into PRODUCT (PRD_DESCRIPTION, PRD_ID) values (?,?);
</statement>
</com:TTextHighlighter>

<div class="note"><b class="tip">Note:</b>
Parameter Map names are always local to the Data Map definition file where
they are defined. You can refer to a Parameter Map in another Data Map
definition file by prefixing the <tt>id</tt> of the Parameter Map with the
namespace of the Data Map (set in the <tt>&lt;sqlMap&gt;</tt> root element). 
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<h2><tt>&lt;parameterMap&gt;</tt> attributes</h2>

<p>The <tt>&lt;parameterMap&gt;</tt> element
accepts two attributes: <tt>id</tt> (required) and <tt>extends</tt> (optional).</p>

<h3><tt>id</tt> attribute</h3>

<p>The required <tt>id</tt> attribute provides a
unique identifier for the <tt>&lt;parameterMap&gt;</tt> within this Data Map.</p>

<h3><tt>extends</tt> attribute</h3>
<p>The optional <tt>extends</tt> attribute can be set to the name of another
<tt>parameterMap</tt> upon which to base this <tt>parameterMap</tt>. All properties of
the super <tt>parameterMap</tt> will be included as part of this
<tt>parameterMap</tt>, and values from the super <tt>parameterMap</tt> are set before
any values specified by this <tt>parameterMap</tt>. The effect is similar to
extending a class.</p>

<h1><tt>&lt;parameter&gt;</tt> Elements</h1>
<p>The <tt>&lt;parameterMap&gt;</tt> element holds one or more parameter child elements
that map object properties to placeholders in a SQL statement. The sections
that follow describe each of the attributes.</p>

<h2><tt>property</tt> attribute</h2>
<p>The <tt>property</tt> attribute of <tt>&lt;parameter&gt;</tt> is the name of a property of
the parameter object. It may also be the name of an entry in an array. The
name can be used more than once depending on the number of times it is needed
in the statement. (In an update, you might set a column that is also part of
the where clause.)</p>

<h2><tt>direction</tt> attribute</h2>
<p>The <tt>direction</tt> attribute may be used to indicate a stored procedure
parameter's direction.</p>

<!-- tabular: align=|l|l|, width=(0.2 0.4) --> 
<table class="tabular">
        <tr><th>Value</th><th>Description</th></tr>
        <tr>
                <td><tt>Input</tt></td>
                <td>input-only</td>
        </tr>
        <tr>
                <td><tt>Output</tt></td>
                <td>output-only</td>
        </tr>
        <tr>
                <td><tt>InputOutput</tt></td>
                <td>bidirectional</td>
        </tr>
</table>

<h2><tt>column</tt> attribute</h2>
<p>The <tt>column</tt> attribute is used to define to the name of a parameter used by
a stored procedure.</p>

<h2><tt>dbType</tt> attribute</h2>
<p>The <tt>dbType</tt> attribute is used to explicitly specify the database column
type of the parameter to be set by this property. This attribute is normally
only required if the column is nullable. Although, another reason to use the
<tt>dbType</tt> attribute is to explicitly specify date types. Most SQL databases
have more than one <tt>datetime</tt> type. Usually, a database has at least three
different types (DATE, DATETIME, TIMESTAMP). In order for the value to map
correctly, you might need to specify the column's <tt>dbType</tt>.</p>

<div class="note"><b class="tip">Note:</b>
Most providers only need the <tt>dbType</tt> specified for nullable columns. In
this case, you only need to specify the type for the columns that are
nullable.
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<h2><tt>type</tt> attribute</h2>
<p>The <tt>type</tt> attribute is used to specify the type of the parameter's
property. This attribute is useful when passing <tt>InputOutput</tt> and
<tt>Output</tt> parameters into stored procedures. The framework uses the
specified type to properly handle and set the parameter object's properties
with the procedure's output values after execution.</p>

<h2><tt>nullValue</tt> attribute</h2>

<p>The <tt>nullValue</tt> attribute can be set to any valid value (based on property
type). The <tt>nullValue</tt> attribute is used to specify an outgoing null value
replacement. What this means is that when the value is detected in the object
property, a NULL will be written to the database (the opposite behavior of an
inbound null value replacement). This allows you to use a magic null number in
your application for types that do not support null values (such as int,
double, float). When these types of properties contain a matching null value
(for example, say, -9999), a NULL will be written to the database instead of
the value.</p>

<div class="tip"><b class="tip">Tip:</b>
For round-trip transparency of null values, you must also specify database
columns null value replacements in your <a href="?page=Manual.ResultMaps">Result Map</a>.
</div>

<h2><tt>size</tt> attribute</h2>
<p>The <tt>size</tt> attribute sets the maximum size of the data within the column.</p>

<h2><tt>precision</tt> attribute</h2>
<p>The <tt>precision</tt> attribute is used to set the maximum number of digits used
to represent the property value.</p>

<h2><tt>scale</tt> attribute</h2>
<p>The <tt>scale</tt> attribute sets the number of decimal places used to resolve the
property value.</p>

<h2><tt>typeHandler</tt> attribute</h2>
<p>The <tt>typeHandler</tt> attribute allows the use of a 
<a href="?page=Manual.CustomTypeHandlers">Custom Type Handler</a>. This allows you to extend the DataMapper's
capabilities in handling types that are specific to your database provider,
are not handled by your database provider, or just happen to be a part of your
application design. You can create custom type handlers to deal with storing
and retrieving booleans from your database for example.</p>

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