DbDescriptor.md

DbDescriptor

The DbDescriptor class is the class that "knows" the structure of a database. A database table or view is represented by the TableDescriptor class, which has the following properties:

TableDescriptor

• {string} name: name of the table or view
• {string} xtype: T for tables, V for views
• {boolean} dbo: true if it is a DBO table (common to all schemas where specified by the database)
• {ColumnDescriptor[]} columns: an array of column descriptors

A TableDescriptor exposes a method getKey that returns an array of key field names for the associated table.

ColumnDescriptor

A ColumnDescriptor describes a single column and contains the fields:

- {string} name        - field name
- {string} type        - type in the database
- {string} ctype       - JavaScript type
- {number} max_length  - size in bytes
- {number} precision   - number of integer digits
- {number} scale       - number of decimal digits
- {boolean} is_nullable - true if it can be null
- {boolean} pk          - true if it is part of the primary key

The DbDescriptor class manages a dictionary of TableDescriptor, which is shared among all connections to the same database.

• createTable(tableName): returns the promise for a DataTable with the structure (columns, key) of the specified table. This DataTable will have the properties maxLenght, allowNull, and cType of the DataColumn set, and the primary key.
• table(tableName, tableDescriptor): reads or sets the table descriptor associated with a tableName. If the TableDescriptor of a table has not been set manually before, it is requested from the database's system tables (which will vary depending on the database).

The DbList module initializes with the init method, which reads the configuration of existing databases. In particular, it is a JSON file that stores a dictionary of the form "db code" => database settings, for example:

{
    "main": {
        "server": "192.168.10.122,1434",
        "useTrustedConnection": false,
        "user": "nino ",
        "pwd": "yourPassword",
        "database": "dbName",
        "sqlModule": "jsSqlServerDriver", 
        "defaultSchema": "amministrazione",
        "persisting": true
    }
}

sqlModule is the name of the SqlDriver to load for this database. It could also be, for example, jsMySqlDriver. The data contained in this file will be passed as is to the constructor of the DataAccess when it is necessary to create a connection to the database.

This file may also contain more than one database description if needed.

Through the getDbInfo function exported by the jsDbList module, we can obtain information about a database. This information is what is read with the init function and may also include other custom fields if desired.

Similarly, with setDbInfo, we can set this information. The changes will also be saved to the file used during init, so we will find them at the next initialization.

With the getConnection(dbCode) and getDataAccess(dbCode) functions exposed by the jsDbList module, it is possible to obtain a physical connection (Connection) or an instance of a DataAccess to the database with a specific dbCode. The Security class associated with DataAccess from the same database is a shared singleton among all DataAccess connected to the same dbCode, for efficiency reasons.

However, we usually do not directly call the getConnection or getDataAccess functions of dbList, but we do it indirectly through the JsConnectionPool class. The latter manages a pool of connections to the same dbCode and exposes a getDataAccess method that returns a JsConnectionPool, which in turn exposes a getDataAccess method that gives the actual DataAccess. It is then possible to release the connection using the release method of JsConnectionPool.

In any case, these operations are normally all performed by the jsApplication class and are usually not used since we already have the database connection in req.app.locals.context.sqlConn (the physical Connection) and req.app.locals.context.dataAccess (the DataAccess).

Deallocation is also done automatically by jsApplication.