## MgDbConstraint

MgDbConstraint — Represents a constraint within a database

## Synopsis

    struct      MgDbConstraint;
    enum        MgDbConstraintType;
    enum        MgDbConstraintFkAction;
    guint       mg_db_constraint_get_type       (void);
    GObject*    mg_db_constraint_new            (MgDbTable *table,
                                                 MgDbConstraintType type);
    MgDbConstraintType mg_db_constraint_get_constraint_type
                                                (MgDbConstraint *cstr);
    gboolean    mg_db_constraint_equal          (MgDbConstraint *cstr1,
                                                 MgDbConstraint *cstr2);
    MgDbTable*  mg_db_constraint_get_table      (MgDbConstraint *cstr);
    gboolean    mg_db_constraint_uses_field     (MgDbConstraint *cstr,
                                                 MgDbField *field);
    void        mg_db_constraint_pkey_set_fields
                                                (MgDbConstraint *cstr,
                                                 const GSList *fields);
    GSList*     mg_db_constraint_pkey_get_fields
                                                (MgDbConstraint *cstr);
    void        mg_db_constraint_fkey_set_fields
                                                (MgDbConstraint *cstr,
                                                 const GSList *pairs);
    MgDbTable*  mg_db_constraint_fkey_get_ref_table
                                                (MgDbConstraint *cstr);
    GSList*     mg_db_constraint_fkey_get_fields
                                                (MgDbConstraint *cstr);
    void        mg_db_constraint_fkey_set_actions
                                                (MgDbConstraint *cstr,
                                                 MgDbConstraintFkAction on_update,
                                                 MgDbConstraintFkAction on_delete);
    void        mg_db_constraint_fkey_get_actions
                                                (MgDbConstraint *cstr,
                                                 MgDbConstraintFkAction *on_update,
                                                 MgDbConstraintFkAction *on_delete);
    void        mg_db_constraint_unique_set_fields
                                                (MgDbConstraint *cstr,
                                                 const GSList *fields);
    GSList*     mg_db_constraint_unique_get_fields
                                                (MgDbConstraint *cstr);
    void        mg_db_constraint_not_null_set_field
                                                (MgDbConstraint *cstr,
                                                 MgDbField *field);
    MgDbField*  mg_db_constraint_not_null_get_field
                                                (MgDbConstraint *cstr);

## Object Hierarchy

      GObject
       +----MgBase
             +----MgDbConstraint

## Implemented Interfaces

MgDbConstraint implements MgXmlStorage and MgReferer.

## Properties

      "user-constraint"      gboolean             : Read / Write

## Signal Prototypes

    "templ-signal"
                void        user_function      (MgDbConstraint *mgdbconstraint,
                                                gpointer user_data);

## Description

Any type of constraint in a database is represented by one instance of the class (see the possible types of constraints). As some constraints may not depend exclusively on a table, all the constraints are attached to the database itself.

It implements the MgXmlStorage and MgReferer interfaces.

## Details

### struct MgDbConstraint

    struct MgDbConstraint;

### enum MgDbConstraintType

    typedef enum
    {
    	CONSTRAINT_PRIMARY_KEY,
    	CONSTRAINT_FOREIGN_KEY,
    	CONSTRAINT_UNIQUE,
    	CONSTRAINT_NOT_NULL,
    	CONSTRAINT_CHECK_EXPR,
    	CONSTRAINT_UNKNOWN
    } MgDbConstraintType;

Possible types of constraints

`CONSTRAINT_PRIMARY_KEY` | Represents a primary key constraint which can be on a single field or on several fields at once.
`CONSTRAINT_FOREIGN_KEY` | Represents a foreign key constraint which can be on a single field or on several fields at once, along with the referenced fields in another table.
`CONSTRAINT_UNIQUE` | Represents a UNIQUE constraint (on a signle field or on several fields at once).
`CONSTRAINT_NOT_NULL` | Represents a NOT NULL constraint on a single field.
`CONSTRAINT_CHECK_EXPR` | Represents a CHECK constraint on any number of fields or tables (not yet implemented).
`CONSTRAINT_UNKNOWN` | For error purposes.

### enum MgDbConstraintFkAction

    typedef enum
    {
    	CONSTRAINT_FK_ACTION_CASCADE,
    	CONSTRAINT_FK_ACTION_SET_NULL,
    	CONSTRAINT_FK_ACTION_SET_DEFAULT,
    	CONSTRAINT_FK_ACTION_SET_VALUE,
    	CONSTRAINT_FK_ACTION_NO_ACTION
    } MgDbConstraintFkAction;

Possible actions performed by the DBMS on various events

### mg_db_constraint_get_type ()

    guint       mg_db_constraint_get_type       (void);

Returns : |

### mg_db_constraint_new ()

    GObject*    mg_db_constraint_new            (MgDbTable *table,
                                                 MgDbConstraintType type);

Creates a new MgDbConstraint object

`table` : | the MgDbTable to which the constraint is attached
`type` : | the type of constraint
Returns : | the new object

### mg_db_constraint_get_constraint_type ()

    MgDbConstraintType mg_db_constraint_get_constraint_type
                                                (MgDbConstraint *cstr);

Get the type of constraint the `cstr` object represents

`cstr` : | a MgDbConstraint object
Returns : | the constraint type

### mg_db_constraint_equal ()

    gboolean    mg_db_constraint_equal          (MgDbConstraint *cstr1,
                                                 MgDbConstraint *cstr2);

Compares two MgDbConstraint objects to see if they are equal, without taking into account the name of the constraints or weather they are user or system defined

`cstr1` : | the first MgDbConstraint to compare
`cstr2` : | the second MgDbConstraint to compare
Returns : | TRUE if the two constraints are equal and FALSE otherwise

### mg_db_constraint_get_table ()

    MgDbTable*  mg_db_constraint_get_table      (MgDbConstraint *cstr);

Get the table to which the constraint is attached

`cstr` : | a MgDbConstraint object
Returns : | the MgDbTable

### mg_db_constraint_uses_field ()

    gboolean    mg_db_constraint_uses_field     (MgDbConstraint *cstr,
                                                 MgDbField *field);

Tests if `field` is part of the `cstr` constraint

`cstr` : | a MgDbConstraint object
`field` : | a MgDbField object
Returns : | TRUE if `cstr` uses `field`

### mg_db_constraint_pkey_set_fields ()

    void        mg_db_constraint_pkey_set_fields
                                                (MgDbConstraint *cstr,
                                                 const GSList *fields);

Sets the fields which make the primary key represented by `cstr`. All the fields must belong to the same MgDbTable to which the constraint is attached

`cstr` : | a MgDbConstraint object
`fields` : | a list of MgDbField objects

### mg_db_constraint_pkey_get_fields ()

    GSList*     mg_db_constraint_pkey_get_fields
                                                (MgDbConstraint *cstr);

Get the list of fields composing the primary key constraint which `cstr` represents. The returned list is allocated and must be de-allocated by the caller.

`cstr` : | a MgDbConstraint object
Returns : | a new list of fields

### mg_db_constraint_fkey_set_fields ()

    void        mg_db_constraint_fkey_set_fields
                                                (MgDbConstraint *cstr,
                                                 const GSList *pairs);

Sets the field pairs which make the foreign key represented by `cstr`. All the field pairs must list a field which belong to the same MgDbTable to which the constraint is attached and a field which belongs to a MgDbTable which is different from the one just mentionned and which is within the same database. The pairs are of type MgDbConstraintFkeyPair.

`cstr` : | a MgDbConstraint object
`pairs` : | a list of MgDbField objects

### mg_db_constraint_fkey_get_ref_table ()

    MgDbTable*  mg_db_constraint_fkey_get_ref_table
                                                (MgDbConstraint *cstr);

Get the MgDbTable at the other end of the foreign key relation represented by this constraint

`cstr` : | a MgDbConstraint object
Returns : | the MgDbTable

### mg_db_constraint_fkey_get_fields ()

    GSList*     mg_db_constraint_fkey_get_fields
                                                (MgDbConstraint *cstr);

Get the list of field pairs composing the foreign key constraint which `cstr` represents. In the returned list, each pair item is allocated and it's up to the caller to free the list and each pair, and the reference count for each pointer to GObjects in each pair is NOT INCREASED, which means the caller of this function DOES NOT hold any reference on the mentionned GObjects (if he needs to, it has to call `g_object_ref()`)

`cstr` : | a MgDbConstraint object
Returns : | a new list of MgDbConstraintFkeyPair pairs

### mg_db_constraint_fkey_set_actions ()

    void        mg_db_constraint_fkey_set_actions
                                                (MgDbConstraint *cstr,
                                                 MgDbConstraintFkAction on_update,
                                                 MgDbConstraintFkAction on_delete);

Sets the actions undertaken by the DBMS when some actions occur on the referenced data

`cstr` : | a MgDbConstraint object
`on_update` : | the action undertaken when an UPDATE occurs
`on_delete` : | the action undertaken when a DELETE occurs

### mg_db_constraint_fkey_get_actions ()

    void        mg_db_constraint_fkey_get_actions
                                                (MgDbConstraint *cstr,
                                                 MgDbConstraintFkAction *on_update,
                                                 MgDbConstraintFkAction *on_delete);

Get the actions undertaken by the DBMS when some actions occur on the referenced data

`cstr` : | a MgDbConstraint object
`on_update` : | an address to store the action undertaken when an UPDATE occurs
`on_delete` : | an address to store the action undertaken when a DELETE occurs

### mg_db_constraint_unique_set_fields ()

    void        mg_db_constraint_unique_set_fields
                                                (MgDbConstraint *cstr,
                                                 const GSList *fields);

`cstr` : | a MgDbConstraint object
`fields` : |

### mg_db_constraint_unique_get_fields ()

    GSList*     mg_db_constraint_unique_get_fields
                                                (MgDbConstraint *cstr);

Get the list of fields represented by this UNIQUE constraint. It's up to the caller to free the list.

`cstr` : | a MgDbConstraint object
Returns : | a new list of fields

### mg_db_constraint_not_null_set_field ()

    void        mg_db_constraint_not_null_set_field
                                                (MgDbConstraint *cstr,
                                                 MgDbField *field);

`cstr` : |
`field` : |

### mg_db_constraint_not_null_get_field ()

    MgDbField*  mg_db_constraint_not_null_get_field
                                                (MgDbConstraint *cstr);

`cstr` : |
Returns : |

## Properties

"`user-constraint`" (gboolean : Read / Write) |

## Signals

### The "templ-signal" signal

    void        user_function                  (MgDbConstraint *mgdbconstraint,
                                                gpointer user_data);

`mgdbconstraint` : | the object which received the signal.
`user_data` : | user data set when the signal handler was connected.