/ External databases / Chapter 43. Links of maps with tables and queries of EDB / Creating and removing a linkIn the case of direct link at the third step of the wizard the user should specify the field of direct link in the table or query of EDB selected before, that is, the field which should contain internal number of the linked feature. The corresponding page of the wizard is shown at figure 43-4.
Figure 43-4. Link creating wizard, page “Selecting direct link field”
ObjectLand offers the user to select a field in the list of integer fields which have precision not less than 10 decimal digits. If there are no such fields in the table/query, an empty list is displayed. It speaks about the impossibility to establish a direct link with the present table or query of EDB.
At the fourth step of link creating wizard (in case of a direct link), the user should select mode of linking and also mode of unlinking. These modes indicate which operations should be performed with value of the field of direct link when a link between a table record and a feature is set or broken. The corresponding page of the wizard is shown at figure 43-5.
Figure 43-5. Link creating wizard, page “Modes of linking and unlinking” for a direct link
The mode of linking is determined by the state of two checkboxes:
Check existence of linked record – if the box is checked, ObjectLand checks the presence of the record in EDB table/query already linked with the feature before setting a link between the feature and the record (that is, it checks the presence of the record with the value of field of direct link equal to internal number of feature). This mode provides control of uniqueness of link between a feature and a record of a table/query.
Check existence of linked feature – if this box is checked, ObjectLand checks the value of the field of direct link before setting the link. If the field contains an empty or a zero value, it is possible to set a link. If the field has a non-zero value, ObjectLand supposes that the present record is already linked with some feature and the link cannot be set.
By default the system checks the presence of a linked feature, but it does not check existence of a linked record.
The mode of unlinking is determined by the state of two other checkboxes:
Put empty value into direct link field – when breaking the link between the present record and the feature, an empty value is entered into the field of direct link.
Put zero into direct link field – when the link between the present record and the feature is broken, zero value is entered into the field of direct link.
If both boxes are checked, the system enters an empty value into the field of direct link, only if this value is supported by the data source of EDB. Otherwise, a zero value is entered. As it was noted above, an empty or a zero value of field of direct link means that the present record is not linked with any feature.
At least one of the two boxes should be checked, because if not, it would be impossible to break the link. By default both boxes are checked.
It should be noted that when a link of a feature type and a table or query of EDB is created, ObjectLand does not check whether the current user has permission to edit the field of direct link. Such a permission is required for the user who performs setting or breaking the link between particular features and records of EDB table/query.
For the case of direct link this step of the wizard is the last one. After pushing the button Finish, the link between the feature type and EDB table/query will be set.
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