{{/* Base constraint template */}}
{{- define "constraint_base" -}}
ALTER TABLE {{qual_table .SchemaName .TableName}}
  ADD CONSTRAINT {{quote_ident .ConstraintName}}
  {{block "constraint_definition" .}}{{end}};
{{- end -}}

{{/* Drop constraint with check */}}
{{- define "drop_constraint_safe" -}}
DO {{dollar_delimiter}}
BEGIN
    IF EXISTS (
        SELECT 1 FROM information_schema.table_constraints
        WHERE table_schema = {{quote .SchemaName}}
          AND table_name = {{quote .TableName}}
          AND constraint_name = {{quote .ConstraintName}}
    ) THEN
        ALTER TABLE {{qual_table .SchemaName .TableName}}
          DROP CONSTRAINT {{quote_ident .ConstraintName}};
    END IF;
END;
{{dollar_delimiter}};
{{- end -}}

{{/* Add constraint with existence check */}}
{{- define "add_constraint_safe" -}}
DO {{dollar_delimiter}}
BEGIN
    IF NOT EXISTS (
        SELECT 1 FROM information_schema.table_constraints
        WHERE table_schema = {{quote .SchemaName}}
          AND table_name = {{quote .TableName}}
          AND constraint_name = {{quote .ConstraintName}}
    ) THEN
        {{template "constraint_base" .}}
    END IF;
END;
{{dollar_delimiter}};
{{- end -}}