Is carbon–sodium bond mainly ionic or covalent?
Electronegativity of Carbon is $2.55$ and that of Sodium is $0.93$ (from Wikipedia) so by the Haney-Smith equation (from J.D. Lee's Inorganic chemistry, third edition) 
$$\%\text{Ionic Character} = 16 |\Delta\chi| =3.5 (\Delta\chi)^2,$$ 
where $\Delta \chi$ is the difference in electronegativity of the two atoms. Carrying out the calculation gives us 
$$\% \textrm{Ionic Character} = 35.1054$$ 
So, it shows that the bond $\ce{C-Na}$ has a less ionic character. 

In Organic Chemistry by Morrison and Boyd we find 


  The carbon-magnesium bond is covalent but highly polar, with carbon pulling the electrons from electropositive magnesium [...].
  But, whatever the metal, it is less electronegative than carbon, and the carbon-metal bond - like the one in the Grignard reagent - is highly polar. Although, the organic group is not full-fledged carbanion [...] it nevertheless has considerable carbanion character.  


Note: No bond is completely ionic or covalent, what matters is which character is dominant and hence the bond will behave like that.