As a supplement at 2021…

Ecto v3 with MySQL < 5.7.8 (don’t support JSON) :

• use :text in migrations.
• use :map or {:array, inner_type} at schema level, and Ecto will encode and decode it for you.

Ecto v3 with MySQL >= 5.7.8:

• use :map or :json in migrations.
• use :map or {:array, inner_type} at schema level.

Some of ecto features are only supported by some adapters - MySQL in general does not support more advanced features, arrays are one of them (this is not a limitation of Ecto, but of MySQL). There are several options of emulating them - either serialising to JSON or simple comma separated string, or even having a separate table.

I had the same problem.
Decided to use an old plain one-to-many association instead. it’s a SQL database in the end.