By Rick Copeland, Jason Myers
Dive into SQLAlchemy, the preferred, open-source code library that is helping Python programmers paintings with relational databases similar to Oracle, MySQL, PostgresSQL, and SQLite. utilizing real-world examples, this functional advisor indicates you ways to construct an easy database program with SQLAlchemy, and the way to connect with a number of databases concurrently with an analogous metadata.
SQL is a robust language for querying and manipulating facts, yet it's tricky to combine it together with your software. SQLAlchemy is helping you map Python items to database tables with no considerably altering your present Python code. If you're an intermediate Python developer with wisdom of easy SQL syntax and relational conception, this booklet serves as either a studying instrument and a convenient reference.
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Extra resources for Essential SQLAlchemy (2nd Edition)
However, this is normally only our execute statement and internal SQLAl‐ chemy code. Typically, the traceback can be ignored for the IntegrityError type. The final block of lines is where the important details are found. It again specifies the type of error, and tells you what caused it. username This points us to the fact that there is a unique constraint on the username column in the users table that we tried to violate. It then provides us with the details of the SQL statement and its compiled parameters similar to what we looked at in Chapter 2.
This is because line_items is actually an association table with some additional data on it between orders and cookies. Association tables are used to enable many-tomany relationships between two other tables. A single ForeignKey on a table is typi‐ cally a sign of a one-to-many relationship; however, if there are multiple ForeignKey relationships on a table, there is a strong possibility that it is an association table. Example 1-3. info Notice that we used a string instead of an actual reference to the column.
As you saw earlier in Example 1-1 and Example 1-2, a column can be made a primary key simply by using the primary_key keyword argument. You can also define composite primary keys by assigning the setting primary_key to True on multiple columns. The key will then essentially be treated like a tuple in which the columns marked as a key will be present in the order they were defined in the table. Primary keys can also be defined after the columns in the table constructor, as shown in the following snippet.