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Import.
author Daniele Nicolodi <nicolodi@science.unitn.it>
date Wed, 23 Nov 2011 19:22:13 +0100
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1 <p>
2 Objects can be retrieved from the repository either by specifying an object ID or a collection ID. The LTPDA
3 Toolbox provides class construtors to retrieve objects. In additon, to retrieve a collection of objects, the <tt>collection</tt> class
4 can be used.
5 </p>
6 <br>
7 <h2>The retrieval process</h2>
8 <p>
9 When an object is retrieved, the following steps are taken:
10 <ol>
11 <li>The object type for the requested ID is retrieved from the <tt>objmeta</tt> table</li>
12 <li>A call is made to the appropriate class constructor</li>
13 <li>The class constructor retrieves the XML string from the objs table</li>
14 <li>The XML string is then converted into an XML Xdoc object</li>
15 <li>The Xdoc object is then parsed to recreate the desired object</li>
16 </ol>
17 </p>
18
19 <br>
20 <h2>Retrieving objects</h2>
21
22 <p>
23 To retrieve an object, you must know its object ID, or the ID of the collection that contains that object. If you don't know the
24 class of the object, you can use the <tt>collection</tt> class as a container, as follows:
25 <div class="fragment"><pre>
26 >> a = collection(plist('hostname', 'localhost', 'database', 'ltpda_test', 'id', 1))
27
28 ** Connection status:
29 LWB: Disconnected from localhost/ltpda_test as hewitson
30 ---- collection 1 ----
31 name: none
32 num objs: 1
33 01: ao | New Block/tsdataNdata=[10x1], fs=1, nsecs=10, t0=1970-01-01 00:00:00.000
34 description:
35 UUID: 9ec7a613-9442-4451-9b40-af18f4afea00
36 ----------------------
37 </pre></div>
38 </p>
39 <p>
40 If you already know the class of the object (for example, <tt>ao</tt>), you can directly call the class constructor method:
41 <div class="fragment"><pre>
42 <span class="comment">% Define the hostname and database</span>
43 hostname = <span class="string">'localhost'</span>;
44 database = <span class="string">'ltpda_test'</span>;
45
46 <span class="comment">% Retrieve the object</span>
47 q = ao(plist(<span class="string">'hostname'</span>, hostname, <span class="string">'database'</span>, dbname, <span class="string">'ID'</span>, 12));
48 </pre></div>
49 </p>
50
51 <br>
52 <p>
53 Multiple objects can be retrieved simultaneously by giving a list of object IDs. For example
54 <div class="fragment"><pre>
55 >> a = ao(plist('hostname', 'localhost', 'database', 'ltpda_test', 'id', [1 2 3]))
56 </pre></div>
57 </p>
58 <br>
59 <h2>Retrieving object collections</h2>
60 <p>
61 Collections of objects can be retrieved by specifying the collection ID. The following script retrieves a collection:
62 <div class="fragment"><pre>
63 >> a = collection(plist('hostname', 'localhost', 'database', 'ltpda_test', 'cid', 1))
64 </pre></div>
65 The output is a <tt>collection</tt> object containing the objects retrieved.
66 </p>
67
68 <h2>Retrieving binary objects</h2>
69 <p>The retrieval process may be speeded up by taking advantage of the fact that the objects are stored in the databases also in binary form.
70 This can be achieved by using the parameter 'binary', that will build the object from the corresponding binary representation, if
71 stored in the database.
72 <div class="fragment"><pre>
73 <span class="comment">% Retrieve the collection</span>
74 q = ao(plist(<span class="string">'hostname'</span>, hostname, <span class="string">'database'</span>, dbname, <span class="string">'ID'</span>, 12, <span class="string">'binary'</span>, <span class="string">'yes'</span>));
75 </pre></div>
76 If the binary representation is not in the database, the object will be built from the xml one.
77 </p>