How to dynamically join multiple tables using SQLalchemy Core?

Question

I have one parent table which holds the primary keys of several child tables. The number of child tables can be arbitrary at run time. Using SQLalchemy core, how can I join multiple child tables to this parent?

Say I have tables of class sqlalchemy.schema.Table with valid FK constraint; how do I construct this query?

I've tried i.e.;

childJoins= [sa.join(parentTable,childTables[0]),sa.join(parentTable,childTables[1])]
# childTables is a list() of Table objects who are guaranteed linked by pk 

qry = sa.select(["*"],from_obj=childJoins)

Which gives;

SELECT * 
FROM 
parentTable JOIN child1 ON child1.P_id = parentTable.C1_Id, 
parentTable JOIN child2  ON child2.P__id = parentTable.C2_Id

So parentTable is listed twice...

Tried many more variations using join() etc. looked at docs, but I still can't get what I want;

SELECT *
FROM parentTable
JOIN child1 ON parentTable.C1_Id=child1.P_Id
JOIN child2 ON parentTable.C2_Id=child2.P_Id 
...
JOIN childN ON parentTable.CN_Id=childN.P_Id

Audrius Kažukauskas · Accepted Answer · 2013-01-26 20:45:46Z

10

Simply chain the joins:

childJoins = parentTable
for child in childTables:
    childJoins = childJoins.join(child)

query = sa.select(['*'], from_obj=childJoins)

answered Jan 26, 2013 at 20:45

Audrius Kažukauskas

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Emily · Accepted Answer · 2017-12-02 05:36:34Z

My solution on multiple table join, inspired by Audrius Kažukauskas's solution above, using SQLAlchemy core:

from sqlalchemy.sql.expression import Select, ColumnClause

select = Select(for_update=for_update)
if self.columns:              # a list of ColumnClause objects
    for c in self.columns:
       select.append_column(c)

# table:  sqlalchemy Table type, the primary table to join to
for (join_type,left,right,left_col,right_col) in self.joins:
    isouter = join_type in ('left', 'left_outer', 'outer')
    onclause = (left.left_column == right.right_column)
    # chain the join tables
    table = table.join(right, onclause=onclause, isouter=isouter)

# if no joins, 'select .. from table where ..'
# if has joins, 'select .. from table join .. on .. join .. on .. where ..
select.append_from(table)

if self.where_clauses:
    select.append_whereclause(and_(*self.where_clauses))
...

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