Use text templates to generate triggers

[andrew-farries]

Refactor the unwieldy Sprintf-based trigger generation code:

https://github.com/xataio/pg-roll/blob/main/pkg/migrations/triggers.go#L54-L78

With an equivalent approach that uses text templates:

package templates

const Function = `CREATE OR REPLACE FUNCTION {{ .Name | qi }}()
    RETURNS TRIGGER
    LANGUAGE PLPGSQL
    AS $$
    DECLARE
      {{- $schemaName := .SchemaName  }}
      {{- $tableName := .TableName  }}
      {{ range .Columns }} 
      {{- .Name | qi }} {{ $schemaName | qi }}.{{ $tableName | qi}}.{{ .Name | qi }}%TYPE := NEW.{{ .Name | qi }};
      {{ end -}}
      latest_schema text;
      search_path text;
    BEGIN
      SELECT {{ .SchemaName | ql }} || '_' || latest_version
        INTO latest_schema
        FROM {{ .StateSchema | qi }}.latest_version({{ .SchemaName | ql }});

      SELECT current_setting
        INTO search_path
        FROM current_setting('search_path');

      IF search_path {{- if eq .Direction "up" }} != {{- else }} = {{ end }} latest_schema {{ if .TestExpr  -}} AND {{ .TestExpr }} {{ end -}} THEN
        NEW.{{ .PhysicalColumn | qi  }} = {{ .SQL }};
      {{- if .ElseExpr }}
      ELSE
        {{ .ElseExpr }};
      {{- end }}
      END IF;

      RETURN NEW;
    END; $$
`

The templates are easier to read, easier to extend and result in cleaner triggers without oddities in indentation levels or empty ELSE blocks when no ElseExpr is provided.

This is in response to this comment.

[exekias]

🤩