generates error constants based on unique indexes

[tlietz]

Closes #355

Let me know if I should change anything.

[stephenafamo]

Why are you using the column names and not the index name?

[tlietz]

Initially, I was using the column names because those were used with the unique constraints.

However, after looking into using index names, the generated constants seem more verbose with Postgres, and information about the columns are lost in Sqlite (assuming default index naming). Should I still go through with using the index name instead of the column name?

Postgres:

// Generated with Index Names
var JetErrors = &jetErrors{
	ErrUniqueJetsPkey: &UniqueConstraintError{s: "jets_pkey"},

	ErrUniqueJetsNameColorKey: &UniqueConstraintError{s: "jets_name_color_key"},

	ErrUniqueJetsPilotIdAirportIdKey: &UniqueConstraintError{s: "jets_pilot_id_airport_id_key"},
}

type jetErrors struct {
	ErrUniqueJetsPkey *UniqueConstraintError

	ErrUniqueJetsNameColorKey *UniqueConstraintError

	ErrUniqueJetsPilotIdAirportIdKey *UniqueConstraintError
}

// Generated with Column Names
var JetErrors = &jetErrors{
	ErrUniqueId: &UniqueConstraintError{s: "jets_pkey"},

	ErrUniqueNameAndColor: &UniqueConstraintError{s: "jets_name_color_key"},

	ErrUniquePilotIdAndAirportId: &UniqueConstraintError{s: "jets_pilot_id_airport_id_key"},
}

type jetErrors struct {
	ErrUniqueId *UniqueConstraintError

	ErrUniqueNameAndColor *UniqueConstraintError

	ErrUniquePilotIdAndAirportId *UniqueConstraintError
}

Sqlite:

// Generated with index Names
var JetErrors = &jetErrors{
	ErrUniqueIdxJets: &UniqueConstraintError{s: "idx_jets"},

	ErrUniqueSqliteAutoindexJets1: &UniqueConstraintError{s: "sqlite_autoindex_jets_1"},
}

type jetErrors struct {
	ErrUniqueIdxJets *UniqueConstraintError

	ErrUniqueSqliteAutoindexJets1 *UniqueConstraintError
}

[stephenafamo]

Yes, let's use the index names.

Users can rename the indexes if they want, and in the future, we can expand aliases to allow aliasing indexes too

[stephenafamo]

Final comment.

Update CHANGELOG.md with your changes, along with attribution (thanks @tlietz)

[tlietz]

Should the docs, or any tests be changed with this pull request?
I'd be glad to update those as well if so.

[stephenafamo]

Should the docs, or any tests be changed with this pull request?

The docs need a lot of work. Any improvements are very welcome. At the moment though, I'm first making sure the CHANGELOG is up to date, I'll find time to dive deeply into improving the docs later.