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# Modes table

The modes table exists to list all the modes available in the model’s network, and its main role is to support the creation of graphs directly from the SQLite project.

Important

Modes must have a unique mode_id composed of a single letter, which is case-sensitive to a total of 52 possible modes in the model.

As described in the SQL data model, all AequilibraE models are created with 4 standard modes, which can be added to or removed by the user, and would look like the following.

## Consistency triggers

As it happens with the links and nodes tables, the modes table is kept consistent with the links table through the use of database triggers.

### Changing the modes allowed in a certain link

Whenever we change the modes allowed on a link, we need to check for two conditions:

- At least one mode is allowed on that link

- All modes allowed on that link exist in the modes table

For each condition, a specific trigger was built, and if any of the checks fails, the transaction will fail.

Having successfully changed the modes allowed in a link, we need to update the modes that are accessible to each of the nodes which are the extremities of this link. For this purpose, a further trigger is created to update the modes field in the nodes table for both of the link’s a_node and b_node.

### Directly changing the modes field in the nodes table

A trigger guarantees that the value being inserted in the field is according to the values found in the associated links’ modes field. If the user attempts to overwrite this value, it will automatically be set back to the appropriate value.

### Adding a new link

The exact same behaviour as for Changing the modes allowed in a certain link applies in this case, but it requires specific new triggers on the creation of the link.

### Editing a mode in the modes table

Whenever we want to edit a mode in the modes table, we need to check for two conditions:

- The new mode_id is exactly one character long

- The old mode_id is not still in use on the network

For each condition, a specific trigger was built, and if any of the checks fails, the transaction will fail.

The requirements for uniqueness and non-absent values are guaranteed during the construction of the modes table by using the keys UNIQUE and NOT NULL.

### Adding a new mode to the modes table

In this case, only the first behaviour mentioned above on Editing a mode in the modes table applies, the verification that the mode_id is exactly one character long. Therefore only one new trigger is required.

### Removing a mode from the modes table

In counterpoint, only the second behaviour mentioned above on Editing a mode in the modes table applies in this case, the verification that the old ‘mode_id’ is not still in use by the network. Therefore only one new trigger is required.

See also

- `aequilibrae.project.network.Modes()`

    Class documentation

- modes table structure

    Data model