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Instead of setting the action manually, you may use .
This opens up the possibility to work on a subset of the data, instead of all associations in the database.
The second one is used to change data directly from your application.
The remaining functions in this module, such as validations, constraints, association handling, are about manipulating changesets.
For example, imagine a Post has many Comments where the comments have IDs 1, 2 and 3.
If you call option in particular must be used carefully as it would allow users to delete any associated data by simply not sending any data for a given field.
As a consequence, validations are always checked before constraints.
Let's fix the age and the name, and assume that the e-mail already exists in the database: Validations and constraints define an explicit boundary when the check happens.
In cases like above, when you want to work only on a single entry, it is much easier to simply work on the associated directly.
For example, we could instead set the is a good fit.
Taking the initial example of users having addresses imagine those addresses are set up to belong to a country.
If you want to allow users to bulk edit all addresses that belong to a single country, you can do so by changing the preload query: Optimistic locking works by keeping a "version" counter for each record; this counter gets incremented each time a modification is made to a record.