database - How to modify field in ecto by Algorithm -

hello i'am looking method change field types , convert values during this.
mean that:

def   alter table(:users)     modify :role, :integer, default: fragment("convertion_function")   end end 

i know ecto.migration#modify/3 function gets &fragment/1 argument. gets 1 argument.
know if possible pass current value &fragment/1 function?
or maybe know better way that?

in postgresql, can done specifying using clause alter table <table> alter column <column> query. couldn't find support in ecto's migration. can use execute execute raw query this. here's how you'd alter posts table's title column string integer , set new value length of original titles:

def   execute """     alter table posts       alter column title         type integer         using length(title);   """ end 

length(title) being fragment expression calculates new value.

you'll want write similar query reverse migration well.

you can read more using in alter table here.