condition for detecting string value in C++ template engine library inja (for SQL insertion of NULL)

Question

using the C++ template engine library

In SQL I can INSERT a row in a database with a VARCHAR SQL type column named 'col1' with

std::string my_template = "INSERT INTO [my_table] (col1) VALUES ('my_value');"

To note that the SQL VARCHAR type requires to single quote

'my_value'

To insert a SQL NULL value, the single quotes are not specified

"INSERT INTO [my_table] (col1) VALUES (NULL);"

I have a template where the value is single quoted, and it allows to insert string values like

std::string insert_template =
"INSERT INTO [my_table] (col1) VALUES ( '{{my_value}}' )";

nlohmann::json json;
json["my_value"] = "some_value";
std::string sql = inja::render(insert_template, json);

calling the template with a tentative insertion of SQL NULL as

nlohmann::json json;
json["my_value"] = "NULL";

is incorrect because the string is single quoted resulting in a SQL insertion of 'NULL' as a 4 character string and not an SQL NULL value

question

how can I make a condition to detect if the argument '{{my_value}}' )

should be single quoted or not in the case the value is "NULL" ?

defining the template as (not single quoted)

std::string insert_template =
"INSERT INTO [my_table] (col1) VALUES ( {{my_value}} )";

would work for a NULL value but not a string

so, I would want something like (in pseudo INJA syntax)

std::string insert_template =
"INSERT INTO [my_table] (col1) VALUES ( 
{% if my_value == NULL %}
{{my_value}} 
{% else %}
'{{my_value}}'
{% endif %}
 )";

Is this possible to achieve somehow?

I wouldn't make "NULL" such a special case, as then you cannot search for this four-character string. Instead, I'd suggest to use std::optional<std::string> to represent nullability and writing a quick wrapper around inja::render() (if it doesn't provide it yet). — lorro
– lorro, Commented Jul 24, 2022 at 15:34

Pedro Vicente · Accepted Answer · 2022-07-28 13:22:13Z

0

this construct in the template chooses the quoted version or not depending on the value being "NULL"

nlohmann::json json4;
  json4["name"] = "NULL";
  std::string template5 = "INSERT INTO [my_table] (col1) VALUES ( \
  {%if name == \"NULL\"%}\
  {{name}}\
  {%else%}\
  '{{name}}'\
  {%endif%}\
  )";

  std::string rendered5 = inja::render(template5, json4);
  std::cout << rendered5 << std::endl;

rendered string is

INSERT INTO [my_table] (col1) VALUES (     NULL    )

answered Jul 28, 2022 at 13:22

Pedro Vicente

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