Add support for destroying a custom field

[HashidaTKS]

GitHub: fix GH-108

If a searchable CustomField is destroyed, corresponding FullTextSearch::Target records are also destroyed.
We can't use existing mapper mechanism for it because CustomField uses
has_many :custom_values, :dependent => :delete_all relation. It doesn't call after_destroy callback that is
needed to synchronize CustomFieldValue and FullTextSearch::Target.

We can remove orphaned FullTextSearch::Target for destroyed CustomFieldValue by
bin/rails full_text_search:synchronize.

[kou]

We should remove entries that refer nonexistent Redmine records instead of not touching them on search.

[HashidaTKS]

Thanks!
I will try to fix current changes to remove entries that refer nonexistent records.

[HashidaTKS]

Question: Do we need to do something in with_project? I think this Mapper is not be used with project because custom field is independent from project.

[kou]

with_project is for mapper that needs JOIN to collect related projects.

[HashidaTKS]

Question: What is not_mapped? When we need this method? Do we need to do some thing in this method?

[kou]

not_mapped is for collecting Redmine records that are not mapped (corresponding FullTextSearch::Target doesn't exist). It's used to synchronize Redmine records and `FullTextSearch::Target.

[HashidaTKS]

There are nothing to do.
So, maybe it is better to not define this class and use FtsMapper directly.

[HashidaTKS]

Concern: This changes don't remove already left fts records. (The error is not resolved only apply this changes, already left fts records should be removed.)

[HashidaTKS]

Concern: This changes don't remove already left fts records. (The error is not resolved only apply this changes, already left fts records should be removed.)

When we should remove the already left (orphan) fts records?
I thinks 2. is better.

1. When migration
2. When startup Redmine
3. When removing a field, which means that removing not only fts records using the field, but also orphan fts records not using the field.

[HashidaTKS]

@kou

Would you review this when you have time?
Do you have any good idea for #109 (comment) ?

[kou]

bin/rails full_text_search:synchronize removes orphan FullTextSearch::Target.