layer-update-one-to-n-relations (#274)

* layer-update-one-to-n-relations

Co-authored-by: Peter Petrik <zilolv@gmail.com>

Author: alex-cit

scripts/wordlist.txt

@@ -217,6 +217,7 @@ uncheck
 undoable
 url
 uuid
+UUIDs
 xcf
 xyz
 yml

src/layer/one-to-n-relations/index.md

@@ -7,53 +7,72 @@ You can clone these projects to take a closer look on 1-N relations:
    - Adding multiple photos to a single feature: <MerginMapsProject id="documentation/forms_multiple_photos" />
 :::
 
-It is often the case that you have a set of spatial features and you want to record some parameters every now and then. For example, there is a GIS layer representing the manholes and the surveyors carry out regular inspections of the manholes. Instead of duplicating the manhole layer and recording each inspection, you can create a non-spatial table and store each inspection as a new row.
+It is often the case that you have a set of spatial features and you want to record their status every now and then. For example, there is a GIS layer representing the manholes and the surveyors carry out regular inspections of the manholes using <MainPlatformNameLink />. Instead of duplicating the manhole layer and recording each inspection as a new feature, inspections can be recorded in a non-spatial table that is linked to the spatial layer. This way, multiple records can be linked to one feature.
 
-The image below shows the manholes locations:
+The image below shows the manhole locations and a form with listed inspections in <MobileAppName />.
 
-![Manhole locations](./input_forms_many-relations_concept1.png)
+![Multiple inspections linked to one point in Mergin Maps Input](./input-1-n.jpg "Multiple inspections linked to one point in Mergin Maps Input")
 
-The manhole point layer has the following attribute table:
+The manhole point layer has the following attribute table: 
 
-![Manhole locations](./input_forms_many-relations_concept2.png)
+| fid | Manhole | Manhole UUID |
+|:---:|:---:|:---:|
+| 1 | 1 | `{70c59616-492e-4757-aa9a-ee61b207ce94}` |
+| 2 | 2 | `{be01b98f-3585-49d4-be74-4cf3530a2989}` |
+| 3 | 3 | `{03178264-0070-45c8-a981-b2474627d7e0}` |
 
-In a separate [non-spatial table](../working_with_nonspatial_data/), we can record the inspections:
+This layer contains only information about the manholes. `Manhole UUID` values are generated using [`uuid()` function as a default value](../attach-multiple-photos-to-features/) when a feature is created. This ensures that these values are **unique** even when multiple surveyors capture new features at the same time. This field will be used to link inspections and manholes.
 
-![Manhole locations](./input_forms_many-relations_concept3.png)
+:::danger
+**Why UUID?** FID can be changed during [synchronisation](../../manage/synchronisation/#synchronisation). As a result, records can end up being linked to wrong features. 
 
-In QGIS, we are using the **Manhole ID** from the point layer and the inspection table to create a 1-N relation between the two tables.
+On the other hand, <QGISHelp ver="latest" link="user_manual/expressions/functions_list.html#uuid" text="UUID" /> (Universally Unique Identifier) is generated to be unique and will not be changed when synced. Therefore, we recommend always using UUID to link layers.
+:::
+
+Inspections are recorded in a separate [non-spatial table](../working_with_nonspatial_data/) with attribute table such as:
+
+| Inspection Date | Blocked? | Flooded? | Inspector  | Manhole UUID |
+|:---:|:---:|:---:|:---:|:---:|
+| 10/05/2022 |  | | <NoSpellcheck id="Joe Schmoe" /> | `{70c59616-492e-4757-aa9a-ee61b207ce94}` |
+| 10/05/2022 |  | :heavy_check_mark: | <NoSpellcheck id="John Doe" /> | `{03178264-0070-45c8-a981-b2474627d7e0}` |
+| 12/05/2022 | :heavy_check_mark:  |   | <NoSpellcheck id="Fred Bloggs" /> | `{70c59616-492e-4757-aa9a-ee61b207ce94}` |
+| 14/05/2022 | :heavy_check_mark: | :heavy_check_mark:  | <NoSpellcheck id="Joe Schmoe" /> | `{be01b98f-3585-49d4-be74-4cf3530a2989}` |
+
+In this table, all information about the inspections are recorded. `Manhole UUID` is filled in automatically based on a 1-N relation that we will set up in QGIS.
 
 The same principle can be used when you want to capture [multiple photos for a single feature](../attach-multiple-photos-to-features/).
 
 
-## Project setup in QGIS
+## How to set up 1-N relations in QGIS project
 You can follow this example by cloning <MerginMapsProject id="documentation/forms_one-to-many-relations" />.
 
+:::tip
+Make sure that your survey layer has a **unique UUID** field to create the link correctly. You will find detailed steps how to set it up in [How to Attach Multiple Photos to Features](../attach-multiple-photos-to-features/)
+:::
+
 To configure 1-N relations in QGIS:
 1. From the main menu, select **Projects** > **Properties ...**
 2. In the **Relations** tab, select  **Add Relation**
-   ![relations](../attach-multiple-photos-to-features/qgis_relation_tab.png)
+   ![Layer properties in QGIS with highlighted Relations tab and Add Relation button](../attach-multiple-photos-to-features/qgis_relation_tab.png "Layer properties in QGIS with highlighted Relations tab and Add Relation button")
 3. A new window will appear, where we can define the parent and child layers and the fields to link the two layers:
    - **Name** is the name of the relation, e.g. `Inspection`
    - **Referenced (parent)** is the spatial layer `manhole_locations`
-   - **Field 1** of the **Referenced (parent)** is the field `Manhole` that contains the ID 
+   - **Field 1** of the **Referenced (parent)** is the field `Manhole UUID` that contains the **unique** UUID 
    - **Referencing (child)** is the non-spatial layer `inspections`
-   - **Field 1** of the **Referencing (child)** layer is the `Manhole ID`, which acts as a foreign key to link inspections to spatial features.
+   - **Field 1** of the **Referencing (child)** layer is the `Manhole UUID`, which acts as a foreign key to link inspections to spatial features
 
-![1-N relations in QGIS](./input_forms_many-relations1.png)
+   ![Add Relation form in QGIS with defined 1-N relation between the parent and the child layer](./qgis-add-relation.jpg "Add Relation form in QGIS with defined 1-N relation between the parent and the child layer")
 
 4. Right-click on the survey layer, select **Properties** and go to the **Attributes** form tab.
 5. Drag and drop the **Inspections** relation to the **Form Layout**.
+   ![Relation added to the Drag and Drop Form Layout in QGIS](./qgis-forms-relations-setup.jpg "Relation added to the Drag and Drop Form Layout in QGIS")
 
-![form](./one-to-n-dnd.png)
-
-This now allows you to add multiple inspections for each manhole location (**manhole_locations** point layer). The inspections records will be stored in the **inspections** table.
-
-When you open the form for an existing record in the **manhole_locations** point layer, you will be able to see the existing inspection records and optionally add, delete or edit the records:
+Now you can add multiple inspections for each manhole location. The inspections records will be stored in the `inspections` table.
 
-![1-N relations in QGIS - form view](./input_forms_many-relations2.png)
+When you open the form for an existing record in the `manhole_locations` point layer, it will display existing inspection records and you can also add, delete or edit the records:
+![Form view of a feature with 1-N relation in QGIS](./qgis-1-N-form.jpg "Form view of a feature with 1-N relation in QGIS")
 
 In <MobileAppName />, the form will look like this:
 
-![Many photos to a single feature](./input_forms_one-to-many.png)
+![Form view of a feature with 1-N relation in Mergin Maps Input](./input_forms_one-to-n.jpg "Form view of a feature with 1-N relation in Mergin Maps Input")