Rails encoding in ASCII-8BIT - ruby-on-rails

Question

I know this have been asked several times, but to me is happening something strange:
I have an index view where rendering certain characters (letters with accent) causes Rails to raise the exception
incompatible character encodings: ASCII-8BIT and UTF-8
so i checked my strings encoding and this is actually ASCII-8BIT everywhere, even though i set the proper encoding to UTF-8 in my application.rb
config.encoding = "utf-8"
and in my enviroment.rb
Encoding.default_external = Encoding::UTF_8
Encoding.default_internal = Encoding::UTF_8
and in my database it appear:
character_set_database = utf-8
as suggestend in some guides.
Strings are inserted with a textarea field and are not concatenated to any other already inserted string.
The strange things are:
this happens only in the index view, whereas this is not happening in the show (same resource)
this happens only for this model (which is an email, with subject and body, but this shouldn't affect anything)
In my development environment everything goes well setting str.force_encoding('utf-8'), whereas in my production environment this is not working. (dev i'm with Ruby 2.0.0, in production Ruby 2.1.0, both Rails4, and both MySql)
setting the file view with # encoding utf-8 also doesn't work
trying str.force_encoding('ascii-8bit').encode('utf-8') says Encoding::UndefinedConversionError "\xC3" from ASCII-8BIT to UTF-8 which is an à, while using body.force_encoding('ascii-8bit').encode('UTF-8', :invalid => :replace, :undef => :replace, :replace => '?'), replaces all accented charaters with a ?, while str.force_encoding('iso-8859-1').encode('utf-8') obviously generates the wrong character (a ?).
So my questions are 2:
- why is rails setting the string encodint to ascii-8bit?
- how to solve this issue?
I've already checked these questions (the newest ones with rails4):
Rails View Encoding Issues
"\xC2" to UTF-8 in conversion from ASCII-8BIT to UTF-8
How to convert a string to UTF8 in Ruby
Encoding::UndefinedConversionError: "\xE4" from ASCII-8BIT to UTF-8
and other resources also, but nothing worked.

Answer 1

You probably have a string literal in your source code somewhere that you then concatenate another string too. For instance:
some_string = "this is a string"
or even
some_string = "" #empty string
Those strings, stored in some_string, will be marked ASCII_8BIT, and if you then later do something like:
some_string = some_string + unicode_string
Then you'll get the error. That is, those strings will be marked ASCII-8BIT unless you add, to the top of the file where the string literals are created:
#encoding: utf-8
That declaration determines the default encoding that string literals in source code will have.
I am just guessing, because this pattern is a common source of this problem. To know more for sure, it would take more information than is in your question -- it would take debugging the actual source code, to figure out exactly what string is tagged as ASCII-8BIT when you expect it to be tagged UTF-8 instead, and exactly where that String came from.