create tables with foreign key

Question

I write a function to create a table:

def createSQLCreateTableCommand(tableName, columns, foreignKeys):
    columns = ["%s"%(c) for c in columns]
    foreignKeys = ["FOREIGN KEY (%s) REFERENCES %s(%s)"%(fk[0],fk[1],fk[2]) for fk in foreignKeys]
    cmd = """CREATE TABLE IF NOT EXISTS %s (%s)"""%(tableName,','.join(columns+foreignKeys))
    return cmd

I wanna test my function and write the following code. "visit" and "test" are just tables with some columns. In table "test_result", it contains a foreign Key to the rowid in the "visit" table and a foreign Key to the rowid in the "test" table. My code works well for "visit" and "test"table but fails to demonstrate the "test_result" table. I am not sure not to express the foreign key argument properly. Anyone can help me to fix my code?

conn = sqlite.connect(":memory:")
cursor = conn.cursor()

sqlVisits = createSQLCreateTableCommand("visit",["visitid text"],[])
sqlTests = createSQLCreateTableCommand("test",["name text","unit text","low text","high text"],[])
sqlTestResults = createSQLCreateTableCommand("test_result",["value text","time_date text"],["visit int","test int"])    #this is not correct

cursor.execute(sqlVisits)
cursor.execute(sqlTests)
cursor.execute(sqlTestResults)
conn.commit()

cursor.execute("""SELECT tbl_name FROM sqlite_master WHERE type = 'table'""")
print cursor.fetchall()

Answer 1

When you print the actual output of the function:

>>> createSQLCreateTableCommand("test_result",["value text","time_date text"],["visit int","test int"])
CREATE TABLE IF NOT EXISTS test_result (
    value text,
    time_date text,
    FOREIGN KEY (v) REFERENCES i(s),
    FOREIGN KEY (t) REFERENCES e(s)
)

You see that expressions like fk[0] extract single characters from strings.

The parameter foreignKeys must be an array of something that consists of three strings: the child column name, the parent table name, and the parent column name:

>>> print createSQLCreateTableCommand("test_result",
...                                   ["value text", "time_date text"],
...                                   [("visitid", "visit", "visitid"),
...                                    ("testname", "test", "name")])
CREATE TABLE IF NOT EXISTS test_result (
    value text,
    time_date text,
    FOREIGN KEY (visitid) REFERENCES visit(visitid),
    FOREIGN KEY (testname) REFERENCES test(name)
)

However, this is not valid SQL because FOREIGN KEY constraints must refer to existing columns in the child table.

To fix this, change the createSQLCreateTableCommand function to create a column with a column constraint for foreign keys:

>>> def createSQLCreateTableCommand(tableName, columns, foreignKeys):
...     columns = ["%s"%(c) for c in columns]
...     foreignKeys = ["%s REFERENCES %s(%s)"%(fk[0],fk[1],fk[2]) for fk in foreignKeys]
...     cmd = """CREATE TABLE IF NOT EXISTS %s (%s)"""%(tableName,','.join(columns+foreignKeys))
...     return cmd
...
>>> print createSQLCreateTableCommand("test_result",
...                                   ["value text", "time_date text"],
...                                   [("visitid text", "visit", "visitid"),
...                                    ("testname text", "test", "name")])
CREATE TABLE IF NOT EXISTS test_result (
    value text,
    time_date text,
    visitid text REFERENCES visit(visitid),
    testname text REFERENCES test(name)
)