Currently, thedocumentation about embedded forms is incorrect. The documentation states that the "cascade_validation" option should be used to enable validation of the embedded form. This is wrong. Instead, one should set theValid constraint on the respective property in the model.

The "cascade_validation" option was introduced to solve a very rare use case, namely when embedded forms have the "mapped" option set tofalse. This use case can also be solved using theValid constraint as of 2.5. Thus "cascade_validation" will be deprecated in 2.6.

More information in the deprecation ticket:symfony/symfony#12237