# Transforming and mapping data

You can change data once it is loaded from an external source, via transformers and mappers.

[Transformers](/data/changing-data/transformers.md) are used to change data values, for example converting a datetime string to a DateTime object.

[Mappers](/data/changing-data/mapping.md) are used to map data to a new data structure so it is more useful for processing, for example converting an array of raw data to a collection of objects.

You can use mappers and transformers together, it's usually more useful to transform data as it's being mapped to a more useful data structure.

Example use cases are:

* Prepare data values (e.g. strip tags, decode HTML entities)
* Rename data fields (e.g. from "person\_name" to "name")
* Update data values to match your local values (e.g. map the category "T-Shirts" to "casual")
* Map a single item to an object (and optionally type set item fields, e.g. to an DateTime object)
* Map a collection of items to a set of objects

We use Symfony's PropertyAccess component to help read and write data. See [how to write property paths](/data/changing-data/property-paths.md) for more details.


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