# Data model structure
## Tree structure
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The data model has a tree structure that facilitates mapping the hierarchical structure of business domain objects.
**Leaves** of the tree represent the most detailed element of the structure. Leaves, called fields, no longer have any subordinate elements (children) and represent a single data field. Examples include "customer name" or "bank account number".
**Nodes** of the tree represent objects composed of other objects and/or fields. Examples: "correspondence address", "customer income".
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Figure 1. Tree structure
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### Nodes
Each data model node has the following attributes:
* a key unique among its siblings (descendants of the same node)
* cardinality expressed by the formula `n..m` meaning from `n` occurrences minimum to `m` maximum, e.g.:
* `1..1` - the object occurs only once
* `0..1` - the object is optional
* `1..null` - the object occurs at least once, with no limit on the maximum number of occurrences
* a description of the object's meaning for the application
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Nodes can therefore be uniquely identified by using a key created by concatenating the keys of parent nodes, e.g.: `client.email`.
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### Leaves
In addition to node attributes, leaves also have:
* data sources - define how and from where the value is retrieved
* default value - what the field will return if none of the data sources returns a value
A field can have a defined list of data sources. Resolving a field value consists of calling the data sources in the order they were defined until a non-empty value is obtained. Thus, the first source returning a non-empty value wins.
We can retrieve data from the model only for selected individual keys, i.e. we cannot retrieve values of keys that have children underneath them, just as we cannot retrieve the entire model in a single request.
## Arrays in the data model
Model nodes marked as multiple (maximum cardinality `>1`) create an array of simple values (for fields) and composite objects (for nodes that are not leaves). It is also possible to define nested collections.
### Keys and identifiers
When referring to data model nodes defined as an array (or being descendants of an array), you must always use array notation:
* array\[].field - retrieves all occurrences of the field; depending on the API used, it will return a collection or a single string with a "," separator
* array\[0].field - retrieves a single value resolved by the collection row indicated in the key (in the example, 0 means the first element of the array),
* array.field - will cause an error; it is not possible to refer to an array node without array notation,
### Defining an array
To define an array field, it is necessary that:
* the multiple data model node must have a cardinality greater than `multiplicityMax=1`, e.g. `1..null`,
* the multiple data model node must have defined sources pointing to an array; the indexes of this collection will be used to index repetitions of model values,
* nodes below the multiple data model node may use, in mapping, the index of the iterated model node to indicate specific elements of the service output,
* the parent node will always expose its current index in the execution context (while resolving values for the collection) under the identifier *`nodeNameIdx`,*
* iterated fields by the parent should point to objects inside the collection over which the parent iterates,
* however, this is not strictly necessary; it is possible to use this index to iterate over a completely different field or to extract fields that are not arrays at all.
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For example, having a service result consisting of a fixed field and two arrays - known to have the same length (arrayA, arrayB, fieldC) - it is possible to create a mapping in which the model iterates over arrayA, but its subfields extract successive fields from arrayB and a fixed value from fieldC.
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## Linking a provider with the data model
A provider is a local data source written in [ScriptCode](https://docs.eximee.com/budowanie-aplikacji/logika-biznesowa/scriptcode/wprowadzenie-do-scriptcode), which defines how values for model fields are obtained. It can be used by fields to supply their values. A data source can be a function:
* script-based,
* calling an external REST service,
* referring to another element of the model,
* retrieving a value from the application configuration.
The provider described below is a **data source for a data model leaf**. It is used to fetch the current exchange rate from an external API (NBP):
* `exchangeRate` is **a leaf (field)** in the tree,
* this field has no children and represents a single value (exchange rate),
* it can be nested in a node, e.g.: `exchangeRates.exchangeRateEUR`.
Figure 2. Example of a provider fetching the current exchange rate from an API (NBP)
How it works:
* the key is taken from the provider's input parameters, i.e. the currency code (`currencyCode`), e.g. "EUR" or "USD",
* this value is passed to the provider from the data model,
* the provider performs a GET request to the configured endpoint `nbpExchangeRate` using a `api.rest.v1.get`,
* path parameters (`pathParams`) build the request URL in the form: `/rates/A/{currencyCode}` (the letter "A" denotes the table of average exchange rates published by the NBP),
* the API response contains the object `body`, which contains the array `rates`, and the first element is retrieved from the array (`rates[0]`), then its field `mid`, which represents the average exchange rate,
* the provider returns an object containing the field `exchangeRate`, the value of this field corresponds to the retrieved exchange rate and can be further used in the data model or form.
### Configuration of data sources for a field
Each field in the data model can define **data sources**.\
The configuration is available by clicking the pencil icon next to the selected field, which opens the data source settings drawer associated with that field. If data sources have already been defined for a given key, their names are visible in the panel - in the example shown, this is `exchangeRateFromNBPProvider`. After selecting a source, it is possible to configure input parameters and output mapping.
Figure 3. Example of using a provider as a data source in the model
In the tab **Parameters** the values passed to the provider are defined.\
In the case described:
* key: `currencyCode`
* value: `EUR`
This parameter is used by the provider to call the external API.
In the **Mapping** section, it is determined which data from the provider's response will be assigned to the field in the model. In this example, the value `exchangeRate`.
The obtained value can then be used in the data model or directly in the form. More about using the data model in the request can be found in the tab [Data model in the interface](https://docs.eximee.com/~/revisions/6RARJsssdz6tDsOideLu/budowanie-aplikacji/interfejs-uzytkownika/formularze/praca-z-komponentami-bazowymi/model-danych-na-interfejsie).
